History of the idiomatic usage of vanilla

Vanilla is often used figuratively and according to Collins Dictionary:

• If you describe a person or thing as vanilla, you mean that they are ordinary, with no special or extra features.

  • ...just plain vanilla couples like me and Tony. The tensions of the Nixon presidency were replaced by the plain vanilla administration of a friendly, middle-aged, middle-class man from the Middle West.

Vocabulary.com suggests that the term vanilla has a “slightly insulting” nuance:

• This word has another, slightly insulting, meaning: a vanilla movie is plain and kind of boring. A vanilla song is on the blah side. Maybe it's because vanilla food tends to be white and plain that we use this word for other things that are bland.

According to Etymonline this connotation is recent and was , apparently, initially used to refer to sex:

• Meaning "conventional, of ordinary sexual preferences" is 1970s, from notion of whiteness and the common choice of vanilla ice cream.

Questions:

1) Was the figurative usage of “vanilla”, as suggested above, first used with sexual reference, and later spread to more ordinary contexts or was it already used in more general context before the ‘70s?

2) Does the term convey a neutral or disparaging, however slight, connotation, when used referring to people, for instance: a vanilla guy, a vanilla couple etc.

Solution 1:

I think that the source of the idea of vanilla as bland and neutral is the expression "plain vanilla"—signifying vanilla as the primary flavoring (of a pudding, ice cream, cake batter, or other dessert). One of the first recorded instances where "plain vanilla' is used in this sense is in Nell Nelson, "Bun's Garden Party," in the [New York] Evening World (July 3, 1890):

Use pitted cherries, red currants and bits of apple, apricot, pineapple, apple and melon. Candied fruits are very nice and half a pound will fairly gem a three gallon can of cream and convert plain vanilla into Neapolitan, EVENING WORLD, Sick Baby cream or any other title you care to bestow.

For some reason "Sick Baby cream" never caught on—but "plain vanilla" as an undistinguished dessert base did. Another early instance of "plain vanilla" recommends using the psychology of food coloring to jazz up simple desserts. From "Red Sugar and Some Ways to Use It," in the [Sydney, New South Wales] Australian Town and Country Journal (November 10, 1900):

Ice-cream also is much more attractive if varied occasionally in appearance, and the first time it is served tinted to a dainty shade of pink there will be a general inquiry as to what gives the peculiarly delicious flavor. Imagination will naturally suspect the presence of strawberries; raspberries, or other fruit. In reality, a small portion of red sugar was the only addition to the plain vanilla cream, the flavor, however, seeming exceptionally fine.

So white is a problem and vanilla is a problem—and the way forward is to introduce additional colors and flavors.

[[UPDATE (June 25, 2018): Ace researcher RaceYouAnytime has found an even earlier instance of "plain vanilla" (actually, "vanilla plain"), with the implication of simplicity and purity made explicit. From "Woman's Pet Tipple," in the Fort Worth [Texas] Daily Gazette (August 31, 1887):

You see, it's this way: The school girls all want strawberry and vanilla mixed, and the dark ones coffee or chocolate, and the blondes they take pineapple or lemon, and the old ladies call for sars'p'rilla, 'cause it’s cooling to the blood, and the girls who come in with fellows want 'just vanilla plain'—kind of innocent and simple, and the young widows always ask for Vichy, with 'a touch of lemon.'

The source of these insights is a waitress who works at a soda fountain.]]

As the website quoted in lbf's answer observes, one early figurative use of "plain vanilla" is in "Willkie Evolves a Plain Vanilla Foreign Policy for Republicans," in Life magazine (October 19, 1942), which, aside from its title, offers this figurative passage:

Whether rightly or wrongly, many a Republican Congressman voted a straight isolationist ticket before Pearl Harbor chiefly because he could not see a realistic solution to the international problem in any statement from the New Deal. The able Congressman John Vorys of Ohio, for instance, while apparently a strict Republican isolationist, has on a number of occasions expressed his open-mindedness toward international solutions, providing only they are real enough to work. And Mr. Willkie's good and generous friend, Congressman Charles Halleck of Indiana, might be another case in point. Certain it is that patriotic Republicans, such as both of these men are, would welcome a plain vanilla Republican solution to the mighty international problem which they must squarely face.

Another early figurative instance again involves political parties, but this time Democrats are "plain vanilla." From "John White Issues Blast at Shivers," in the Sweetwater [Texas] Reporter (January 8, 1954):

The only state official who didn't run on both the Democratic and Republican tickets last fall said Thursday night "cleanly - drawn" political battlelines would separate "a lot of good, plain, vanilla Democrats" from "a few raspberry Republicans."

Agricultural Commissioner John C. White, 29. devoted most of his speech to Tarrant county Democrats to criticism of the Texas' system of cross-filing and political hybrids.

Then comes "plain vanilla" in the sense of "garden-variety" or "dime-a-dozen" athlete. From "Hoot Gibson Proves Grit, Determination Pays Off," in the [Abilene, Texas] Optimist (September 17, 1954):

When February ro[l]led around and spring drills, Herb checked out his uniform and went through some more hard bumps.

After Spring drills were over he was more or less still classified as a plain vanilla scrub.

And again in the context of sports, from "After-Game-Party Held; What a Game And What a Party," in the [Abilene, Texas] Brand (February 28, 1958):

The basketball victory (81–62) played second fiddle to the school spirit displayed by both Midwestern and the Cowboy cheering section.

What would normally have been a plain vanilla game burst into a wide open affair mostly because of the cheering fans.

In short order, "plain vanilla" became a descriptive term for all sorts of inedible things—to the human beings who serve on censoring boards (in the Crosbyton [Texas] Review, August 7, 1958); to theatrical entertainment (in the [Abilene, Texas] Optimist, November 20, 1959); to low-cost automobiles (in the [Fort Hood, Texas] Armored Sentinel, May 27, 1960); to electric ranges (in the Crosbyton [Texas] Review, February 8, 1962); to a shirt color (in the [Denton, Texas] Campus Chat, March 23, 1962); to teachers (in the Tulia [Texas] Herald, April 12, 1962); to everyday people (in the [Denton, Texas] Campus Chat, October 10, 1962); to football formations (in the Crosbyton [Texas] Review, October 11, 1962); to (finally) sex education, although not sex itself (in the [Philadelphia, Pennsylvania] Catholic Standard and Times (June 18, 1965).

The predominance of Texas periodicals in the preceding paragraph is not a sign that Texas was a hothouse of "plain vanilla" idiomatic experimentation; it just underscores how limited other states' accessible post-1920s newspaper databases are. The Portal to Texas History archive is restricted to small-town newspapers and college newspapers—no big metropolitan dailies with their many copyrighted and syndicated stories from wire services and prominent national newspapers—but it still manages to host a respectable amount of relatively recent searchable content.

In any event, it seems unlikely to me that the writers of these various instances from the period 1942–1965 were taking their cue on "plain vanilla" (knowingly or not) from the world of sexual preferences.

As I noted earlier, the original idea of "plain vanilla" was not that it was unappealing in itself, but that it could serve as a base for a multitude of interesting variations through the addition of other ingredients. Inevitably, however, the relative simplicity of "plain vanilla" invited critical dissatisfaction based on the predictability and unimaginativeness of "plain vanilla." This pejorative element certainly lingers as an overtone of describing something or someone today as "vanilla."

Solution 2:

1. "Was the figurative usage of “vanilla”, as suggested above, first used with sexual reference, and later spread to more ordinary contexts or was it already used in more general context before the ‘70s?"

It appears its figurative use appeared earlier:

Lexiculture: vanilla
A LIFE magazine article from 1942 provides an example of vanilla being used to describe something other than flavor. The article was titled “Willkie Evolves a Plain Vanilla Foreign Policy for Republicans.” It is important to realize that the phrase “plain vanilla” is being used in a very popular magazine that denotes and captures much of what is going on in the world and also popular American culture. The phrase “plain vanilla” would not have been chosen if the readers of the magazine were not familiar with the descriptive choice of words. It can be inferred that vanilla began to shift from a description of an actual flavor to something meaning plain or boring before the 1940s.
Glossographia

2. "Does the term convey a neutral or disparaging, however slight, connotation, when used referring to people, for instance: a vanilla guy, a vanilla couple etc."

Yes, it does have pejorative uses ... and context is necessary:

Vanilla has a PR problem. As a noun, vanilla refers to our most fragrant and complex flavor. But as an adjective, it is a pejorative. Slate Mag: THE WHITE STUFF

.

Solution 3:

vanilla

Etymonline is never really a secure source, but in this case it is copying directly from the OED. They still haven't added an adj. sense to their entry on 'vanilla' but they have a draft addition pending since 1997:

[ < the popular perception of vanilla as the ordinary, bland flavour of ice-cream.] Plain, basic, conventional; (esp. of a computer, program, or other product) having no interesting or unusual feature; safe, unadventurous. Used orig. with reference to sexual activity (esp. in vanilla sex). Only occasionally as predicative.adj. colloq. (orig. U.S.).

The earliest cites they list are from Bruce Rodgers's 1972 The Queens' Vernacular: A Gay Lexicon, p. 184 & 205:

Vanilla bar, a gay bar that is not SM.

Vanilla,... rigid, conforming, goody-goody ‘This neighborhood is too vanilla for the licks [sic] of us.’

One thing I did not expect was that the etymology apparently comes through Spanish and French from Latin vagina...

plain-vanilla

Oh, having looked around a bit more, the OED does have a separate subentry for 'plain-vanilla' under the heading 'plain, adj.'

plain-vanilla adj. fig. (orig. U.S.) having no special or additional features; basic, ordinary...

The earliest cite it offers for this sense is a Washington Post article from July 13, 1959:

After Cococabana [sic], Bethany Beach will always look slightly plain vanilla to me.

In any case,

1) The use of 'vanilla' for bland sex grew out of existing use of the much older phrase 'plain vanilla', which was already being used to describe electronics by a 1975 Forbes article.

2) It remains essentially a synonym for 'plain'. It'll usually be pejorative, because of the accentuation of the normalcy and easy availability of replacement. A vanilla guy might be just what someone is looking for, but calling him 'vanilla' emphasizes that he's garden variety and dime-a-dozen. At the same time, a vanilla or plain vanilla stock swap might be preferable to anything too complicated, where the lawyers would end up eating most of the additional profit.

Solution 4:

The context of "Vanilla" as plain, or boring, or basic, or simple, or "base material to which colors & flavors are added" did not originate with sex. Sex just got this adjective applied to it as did everything else.

Real vanilla is very expensive and has a very strong, distinct flavor that cannot be matched by the fake food chemical substitutes. Bad "vanilla" ice cream will either not use ANY vanilla at all, or will use fake vanilla flavorings and dilute them so much that even the fake stuff cannot be tasted. Thus, the difference between PLAIN and VANILLA is dramatic. Over time the "vanilla" label got stuck on products that were actually just "plain" to create a false impression that at least one expensive ingredient was used, even if only a little bit, when no such expense was spent at all.

SO, the term "vanilla" as a pejorative depends HEAVILY upon the context in which it is used, and whether or not the properties of "plain" are a positive or negative in the area under discussion, and whether or not the speaker or audience is aware of the complex differences between "vanilla" & "plain". Even if the intended use is pejorative, the denigration is only a slight one.

Solution 5:

In some simple research, the phrase would seem to have arisen when the synthetic version, Vanillin became the most available and popular flavoring used. That seems to have happen in the late 19th century in Europe, and the 1930's in the U.S.

Real Vanilla is the world’s second most expensive spice/flavoring. Saffron being number one. Of all the flavorings used, vanilla has become one of the most popular around the world.

In the late 19th century, synthetic Vanillin began to be available in wide distribution, making vanilla flavoring more accessible than chocolate. This made vanilla flavored products, made with vanillin, a popular choice.

In the 1930’s, a new process for making vanillin made it much cheaper and it became a staple in every kitchen. It became the most common flavor/spice available. Real vanilla was replaced by ‘plain’ vanillin. People began to be over exposed to it as their main choice for flavored treats: i.e., cakes, cookies, puddings, ice cream, frosting, donuts, etc.

Plain Vanilla began to denote something with very little imagination or work involved. But, since vanilla is still a very popular and expensive flavoring spice, it really is related to vanillin, the synthetic version in every spice cabinet.