In a recent camapign, we ran up against a few vampires. Killing them was easy, it was keeping them that way that was the problem. We came up with this solution: Once we had reduced the vamp to 0 hp and it assumed its mist form, one of the casters would cast wind wall to contain the mist. The Cleric would cast lesser planar ally (IIRC) summoning a Djinn. The Djinn has create object or something similar as a spell like ability. We would request the Djinn create a glass globe around the mist trapping it inside. The Djinn would then take the globe (with vampy inside) back to the plane of air (could substitute the positive energy plane). In our campaign at least, the plane of air is always bright and sunny. Even if somehow the vampire was able to escape before the sunlight did him in, the vampire must get back to its coffin within 1 hour or it disipates, effectively destroying it. Worked well for us but as always YMMV.