what I want to know is if a vampire can choose to go gaseous and leave items behind
No. Since, once gaseous,
The subject and all its gear become insubstantial
As a standard action, a vampire can assume gaseous form
there's nothing stopping a vampire from dropping items before they become gaseous, and for held items, since
Dropping an item in your space or into an adjacent square is a free action.
this could be done in one turn. But, as you asked,
such as armour or other worn items
this would take the requisite action(s) before the standard action to become gaseous.
While it certainly seems reasonable sometimes to assume certain tasks are plausible, you have to take into account the action paradigm in the game. Any action, even non-actions, still occur in time order. Attacks of opportunity, immediate actions, talking as a free action, and completing a readied action are exceptions that can interrupt the action order, but even then, within the flow of combat, there is a strict order. It can seem a bit awkward at times, but overall the concept of turns and actions prevents things like this from happening, where an actor can't simply perform a series of tasks without the possibility of interruption. It is sometimes tempting as a DM to allow exceptions, but in the end, it will make the game less predictable and ultimately unfair.