I have two offers for a Visiting Assistant Professor position. Can I negotiate for a higher salary for a VAP position?
You can certainly try to politely negotiate salary, but I wouldn't count on getting much if anything at all.
At my institution, my experience has been that the administration simply won't negotiate on salary. For a VAP, the department is simply looking for someone who can handle teaching load for a year or two. There are typically many qualified applicants with enough teaching experience, and because the position is temporary there just isn't any point in paying more than necessary.
Brian is right that you're unlikely to make much headway. I think maybe he isn't recognizing the variety of positions that go under VAP (some of whom have serious research expectations), but still, I expect they will have relatively set salaries; it could a lot of work and bureaucracy for the chair to request the change even if s/he wants to. It's often a bit easier for chairs to sneak other things into offers. You might be more likely to be successful if you asked for, say, 1k in research funds rather than salary, since often those funds are more flexible.
It may depend, to some extent, on how much the university wants you, and on how much flexibility they have. If the school doesn't often hire visiting professors, there is more chance they will treat your hire like any other hire (although not tenure track) with some room for negotiation. If they hire numerous VAPs each year, they are more likely to think of it as a more routine kind of hiring with fixed parameters.
Here is a personal anecdote. I had a one year VAP at one point. That year, I also had the option of remaining at my PhD school as an instructor, which would have been less ideal but which would have certainly been cheaper. I managed to convince/negotiate with the other school to raise the salary slightly to compensate (not more than a couple thousand, plus some moving expenses). But, I believe they really wanted me to come.
One thing to keep in mind is the BATNA on both sides. For example, do you have alternate positions available? Does the school likely have equally qualified candidates to choose from if you don't accept the offer? There was a news story last year about a tenure-track job offer that was derailed by negotiations that the university thought were too aggressive, so it may help to inquire informally about how much flexibility there is on the salary.