I know that it is permissible for a company not to pay for unused vacation time at all no matter what the situation. I have seen companies that had a policy that said that. But I think this is probably a state law issue.
If your policy is to normally pay them, then I don't know the answer. It would be a good idea to have such a statement in your policy.
I know that employment-related lawsuits can be quite expensive to defend against even if you win. Getting the advice of an attorney would be a good idea. Unless you have a signed admission of guilt, I think you could have a lot of headaches.
To me, it seems like something that probably doesn't happen very often and I'd probably pay the vacation and be glad I'm rid of the person rather than worry about any potential headaches due to lawsuits for wrongful termination, discrimination, slander, etc.