So what do you propose to do about unresponsiveness?

Flag as "needs author's attention" -- if the author adds additional information, the post can then be reopened

@sam: That gets back to the problem of us being unable to vote to close. That's really the root of this. The correct response to a vague question is to ask for clarification, then vote to close immediately. That way the proper thing is done (assuming enough others vote to close too) automatically if the OP never responds. If he does, all the close-voters and commenters should be notified, allowing for the opportunity to retract the close votes and possibly answer the question.

Flagging a mod feels wrong for this, and then it relies on the mod agreeing with me before my vote takes effect. Downvoting is the only real tool we have been given.

In this particular case, I probably would not have voted to close because the question didn't seem that bad, and I wasn't confident in my interpretation of it. That only leaves a downvote to deal with the unresponsiveness.

The way I'd handle unresponsiveness is the same way I'd handle any problematic behavior of a user as opposed to a post that doesn't rise to moderator intervention and eventual banning. The first few times I'd comment with what the problem was, why it's a problem, and some actionable advice to mitigate it. In this case it might be something like: "You didn't respond to a request for feedback in over two days. This gives the impression that you just dumped the question here and don't really care which is demoralizing to potential answerers and makes them not want to interact with your questions. In the future, try to stay on top of a question in at least the first few days where most feedback is likely to be requested." After giving such advice a couple of times and seeing no improvement, I'd do what it says. I'd start ignoring the users questions. This avoids burnout on my side and disincentivizes their behavior proportionally to how many people agree with me and follow the same policy.

Really, none of the approaches mentioned so far (short of banning) actually directly address the issue. Arguably, one of the main reasons we downvote questions is to communicate to other users that a question isn't worth their time. A close vote on a question asked weeks ago won't help when a new question is asked. A more direct solution (that's no doubt rife with opportunities for abuse and unintended consequences) would be to have something like a multi-factor 5-star rating system for users. You could imagine a rating 1-5 stars in aspects like "responsiveness", "handles feedback well", "communicates clearly", and others. These would then be available by hovering over a user card or something, and users can get an idea of what they are getting into with another user without needing to audit their posting history. There doesn't need to be any mechanical consequence to these ratings.