Ran into a situation this weekend where we were fighting some vampires. The vampire was reduced to zero hitpoints and assumed gaseous form. Per the PRD the vampire defensive ability says this:
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. (It can normally travel up to 9 miles in 2 hours.) Additional damage dealt to a vampire forced into gaseous form has no effect.
So, while the vampire would take no additional damage, can they still be destroyed by a disrupting weapon?
Disruption: A disruption weapon is the bane of all undead. Any undead creature struck in combat must succeed on a DC 14 Will save or be destroyed. A disruption weapon must be a bludgeoning melee weapon.
Since it is just being struck in combat?
I'd also say yes. The disrupting effect doesn't need the weapon to deal damage (to get by the DR of 10/magic bestowed by Gaseous Form) to cause its effect from what I read.
Of course vampires being intelligent would have a route(s) to their coffin(s) picked in advance and if I were them at least a portion of the route would be difficult if not impossible to follow while not Gaseous (and guarded and warded).