Commas before not? [closed]

Solution 1:

The comma rules used here have nothing to do with "not". In your first example sentence, a pair of commas surrounds a nonrestrictive appositive (with a couple of words elided):

I want a vanilla cake this year, not [[a]] chocolate [[one]].

The second comma is absorbed into the terminal period, of course.

In your second example sentence, a pair of commas surrounds a nonrestrictive adverbial clause ("because I wanted to"). (The "not" and "though" can be interpreted as adverbs modifying the entire clause, modifying the verb "wanted", etc.)

"everything following the comma here sounds like it could be its own sentence as well": Actually, the text following neither comma could be its own sentence.

The dash in your last example is fine, although (as you mention) it does suggest greater emphasis. When we surround nonrestrictive information, we typically use pairs of commas, parentheses, or M-dashes.

By the way, there are several other ways in which to interpret these sentences. This is just one possibility.