First, the vampire will lose its hit points and be forced into gaseous form (yes, it's already in gaseous form. This doesn't actually matter). Then it will die, not from holy water but because it is unable to reach its coffin, after two hours:
A vampire heals 5 points of damage each round so long as it has at least 1 hit point. If reduced to 0 hit points in combat, it automatically assumes gaseous form and attempts to escape. It must reach its coffin home within 2 hours or be utterly destroyed. (It can travel up to nine miles in 2 hours.) Any additional damage dealt to a vampire forced into gaseous form has no effect.
However, as Holy Water deals either 1 or 2d4 damage and deals that damage once, not constantly, the vampire is unlikely to be significantly harmed unless you have a method of flushing the area with new holy water each round. If you do, it's not the vulnerability to Holy Water that matters, just the vulnerability to water in general:
immersing a vampire in running water robs it of one-third of its hit points each round until it is destroyed at the end of the third round of immersion.
Which results in a dead vampire real fast, instead of after 2+hours.
As for the gear, see: What happens to vampire possessions when he's destroyed in mist form?
(the answer is unclear because Vampires can't actually use their Gaseous Form ability as written).
