Often, the situation is well-defined enough so that you can still write a solid and informative answer even if the question uses many vague terms. Under those circumstances, just go answer.
An example of such a question would be Fabian's CFL question. It uses vague terms like "harmless" and "dangerous" but the situation is limited enough so that you can write a good answer covering the health risks in the situation he's describing.
If it's so vague it would take mindreading to guess what is asked, or if the question is so generic it could be split in several questions, ask the user to be more specific. If he or she complies, then all is good. If the users does nothing, vote to close the question as "not a real question":
not a real question: It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form.
A good example of a question that is simply unanswerable is “E Numbers” - harmful or not? It's so generic and unclear that it's impossible to tell where to start. The high number of answers represent that well.
Don't just edit the question, though. I think that'd be a misuse of the edit privilege.