This question is way too vague
Why not just: "This question is too vague"? What's the meaning of 'way' in this sentence?
In American English, way is also used informally as an adverb of emphasis. Relevant definitions, according to the Oxford Dictionaries:
Thus, in your example, way is used to emphasize the vagueness of the question. You could just as well say:
Way is acting as an intensifier. Saying that something is way too vague indicates that is more vague than if you had simply said it is too vague.
In this context it is an informal synonym for much, i.e. "This question is much too vague."