"If reduced to 0 hit points in combat, a vampire assumes gaseous form (see below) and attempts to escape. It must reach its coffin home within 2 hours or be utterly destroyed. [...] Additional damage dealt to a vampire forced into gaseous form has no effect."
Does this mean it is impossible to harm a vampire that has been forced into gaseous form, or simply that it stops taking damage once it hits 0, but can then be damaged as normal (per the rules for the spell "Gaseous Form") by subsequent attacks?
I'd presume the gaseous vampire is immune to damage, as undead generally cannot drop below 0.
An interesting question is: what happens to a vampire in gaseous form if healed by negative energy? Do they still need to flee to their coffin?
Vampires are not killed by HP loss. There is only four ways to kill a vampire. One is by exposing it to sunlight for two rounds, two is by immersion in running water for three rounds the third is by severing it head and anointing it with holy water after it is helpless. The finial way to kill a vampire is to prevent it from reaching its coffin when it has been reduced to 0 hp.
Undead are usually killed when they reach 0 HP. Instead of being killed vampires are forced to flee. Since healing does not work after a creature has been killed logically you cannot heal a vampire once it has been forced to assume a gaseous form and flee. Once a vampire has been forced to assume a gaseous form they gain a uniquely vampire condition of near death. If they do not rest in their coffin for an hour within 2 hours of gaining this condition they progress to full death. Nothing besides resting in their coffin can remove the condition.
Mysterious stranger, I just want to clarify that I think as long as the vampire makes it to their coffin before the 2 hr time limit is up, and then rests in their coffin for an hour returns to 1 hp. Your statement sort of made it sound like the have to get to their coffin within 1 hr, so that they have 1 hr (before the end of the 2hr duration) to get their 1 hp back.
Yea as long as they reach the coffin before the two hour deadline they are good, Ok maybe not good, but you get the idea.