That is, if it can when gaseous form is in effect...
If what can kick in? Fast healing? Normally FH is active when a vamp is in gaseous form. But it's not active when it has less than 1 HP. Vamps just have a special provision about when they hit 0 HP's, they must rest in their coffin for an hour to heal 1 HP, at which time they begin fast healing again.
At least that's my understanding of the matter.
PS - I'm the DM in this case, thus why I'm seeking for confirmation on how it works.
Q 148: Suppose a vampire gained a spell-like ability to use telekinesis at will. Can he use it while in Gaseous Form? Gaseous Form says you lose any supernatural abilities, but says nothing about spell-like abilities. It also specifies you may use spells if they are Still, Silent, and you have Eschew Materials. Spell-like abilities don't have any components... (Excluding Warlock, of course.)