Disintegrate is always a crowd pleaser against the undead. You'd need to ask the guy running things what he thinks about this, but it "might" bypass the vampire's escape and recovery attempt. To be more accurate, it
does, but plenty of people don't like this and try to talk their way around it, so you should find out if the DM is one of them.
Any creature reduced to 0 or fewer hit points by this spell is entirely disintegrated, leaving behind only a trace of fine dust.
A trace of fine dust does not have the Fast Healing Ex special quality, nor the Gaseous Form Su special quality. Doesn't have Deathless Frenzy either, so it's a good way of putting down frenzied berserkers that are only still on their feet because of that ability.