Are Visiting Assistant Professor salaries negotiable?

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.

Ben has precisely shown the issue, funds for these positions are difficult to change. If you politely query research or accommodation funds however you may find they are able to allocate from the department budget or various grants as well as pre-purchase items such as laptops (within reason).