I think people are making this too difficult. A non-PC, non-DM special monster dies at zero hit points. Vampires have a special ability that says that when they hit zero the first time, they go misty. This doesn't make vampires special in or have different rules in any other way, it just says that they get a one time extra chance at zero. If they take more damage after that, they're out of extra special chances and die like all the other monsters at zero. Yes, it would be clearer if they reset back to 1 hitpoint when they go misty, but even as is you don't need to make up other rules to make vampires work as written -- they die when they take more damage while misty.