The vampire returns to solid form ten minutes after he assumed gaseous form, and so does his stuff... Unless the vampire decided otherwise prior to being destroyed.
The vampire's gaseous form ability reads,
As a standard action, a vampire can assume gaseous form at will as the spell (caster level 5th), but it can remain gaseous indefinitely.
Take careful note of the word "can" in that quote. The duration of the gaseous form spell doesn't become "Permanent (D)." Instead, the vampire can remain in gaseous form indefinitely: Doing so is explicitly an option that the vampire can choose not to take, if he wants. Thus, unless a vampire deliberately decides to remain in gaseous form indefinitely, its gaseous form has the normal duration of the gaseous form spell: Two minutes per caster level or until dismissed. (Presumably making the duration indefinite doesn't remove the "or until dismissed" detail, or every vampire who elected to remain in gaseous form indefinitely would lose the option of becoming solid.)
Exactly when the vampire has to decide whether to use the spell's normal duration or to remain in gaseous form indefinitely is not explicitly stated in the supernatural ability description, and there are no rules that state when a character must declare which optional behaviour of a supernatural ability they're electing to use when the supernatural ability isn't a spell-like ability but nevertheless mimics the effect of a spell with optional modifications that aren't part of the spell description, so your group's DM will have to make a ruling. There are two possibilities: The vampire decides whether to use the ability's normal duration upon using the ability and assuming gaseous form, or the vampire can decide to extend the duration of his gaseous form ability indefinitely at any point after using it.
In any case, if the vampire elected to remain in gaseous form indefinitely prior to being destroyed, it will remain in gaseous form indefinitely: A dead or destroyed creature is no longer able to dismiss its own ongoing dismiss-able effects, and so the effect will last until it is ended by some external effect.
(Personally, I'd house-rule that a vampire who elects to remain in gaseous form beyond the normal ten minute duration immediately reverts to physical form upon being destroyed, just because it feels less weird than the "ten minutes or forever" thing. Other DMs may rule differently, though.)