Charles Rampant
Adventurer
Remember also that a party that really wants to will try to use a spell or ability that creates Sunlight, which also instantly destroys a Vampire at 0 HP, no mist-form allowed. It might be simpler to assume that the 0 HP - turn to mist - flee thing is actually unlikely, and instead would be a 'failure state' of sorts for the party. (Just like, uhm, you wouldn't assume that a Bad Knight would automatically be able to flee the party, but him fleeing might well be something that happens if the party does bad enough in the combat to not kill him outright or prevent his flight.)
If the plot really requires that the Vampire turn to mist and escape, I suggest you instead have the Vampire do something like Simulacrum, where it isn't really him/her that the party are fighting, but only a mirage or puppet. That way you get your boss fight, the party can win it fair and square, and you still get to keep your boss monster for another part of the storyline.
If the plot really requires that the Vampire turn to mist and escape, I suggest you instead have the Vampire do something like Simulacrum, where it isn't really him/her that the party are fighting, but only a mirage or puppet. That way you get your boss fight, the party can win it fair and square, and you still get to keep your boss monster for another part of the storyline.