This is too vague a term [duplicate]

Solution 1:

Just as the construction vague enough means 'so vague that S'

• The term was vague enough to describe any book. ==>
• The term was so vague that it described any book.

the construction too vague is its negative, meaning 'so vague that Not S'

• The term was too vague to describe the book well. ==>
• The term was so vague that it didn't describe the book well.

If the S is not specified, as in the original example, it remains to be interpreted from context by the addressee. Since vague is not a clear concept, what this means in practice is 'I don't understand'.

The construction in the original example is somewhat more complex,

• This is too vague a term.

instead of

• This term is too vague.

Both are grammatical, but if you want the too Adj construction to be an attributive adjective (i.e, if you want it to modify a noun instead of being a predicate adjective), it must come before a full Noun Phrase, with determiner if any. So you can't say

• *This is a too vague term.
• *Maru is in a too small box

Instead, you have to put too vague before the a, even though it looks weird.

• This is too vague a term. ==>
• This is so vague a term (that s.t. bad happens or s.t. good doesn't happen)

(Note that this rule also applies to so: i.e, it should be so vague a term as well)

Solution 2:

Disclaimer: These are my theories. I'm not a linguist.

too small a box to fit
too proud a man to admit his shortcomings
too nice a person

The phrases in question are adjective phrases, i.e phrases that are headed by an adjective.

"Too" is an adverb modifying the head adjective. The noun phrases "a box" or "a man" are a mysterious part of the structure of adjective phrases that is not understood. They seem to function as an optional expletive to refer to the thing the adjective is modifying. They aren't necessary, you can always remove them.

In your example, saying "this is too vague", although perfectly grammatical, might be ambiguous if the listener did not know what "this" was. So you can optionally add "a term" to be more explicit.

This is too vague a term.

So, it's like a way to restate what the adjective is modifying.