DH has actually done this twice--we had vacations scheduled before he changed jobs. He was upfront and mentioned it during the interview process. I can see waiting until after getting an offer, but understand that it may be a deal-breaker for the company. I would recommend saying that you'd be willing to postpone or cancel the trip--I'm sure you don't want to, bu tin this job climate, you want to be seen as a good job prospect.
In both cases, DH took the time off without pay. I can't imagine a company giving you PAID vacation just a few months after you start, but it would depend on the comapny's vacation policy, I guess. Certainly, we would never expect that he'd get paid.
I also agree with looking at the overall picture for the company--if they have busy seasons and you want to vacation during that time, it's probably not going to be allowed.