The vampire has the following trait:
Misty Escape. When it drops to 0 hit points outside its resting place, the vampire transforms into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious, provided that it isn't in sunlight or running water. If it can't transform, it is destroyed.
Even though a vampire's regeneration doesn't function if the vampire has taken radiant damage since its last turn, there is no similar restriction on misty escape. So I think that if a vampire gets reduced to 0 hp by radiant damage not caused by sunlight (e.g. a paladin's smite, holy water), they can still misty escape. If this is incorrect, please address it in your answer.
Sunburst does radiant damage, and is also sunlight. Its duration is instantaneous, so the sunlight goes away right after doing damage to everyone in the area of effect. If sunburst reduces a vampire to 0 hp, is the vampire "in sunlight" at the moment of taking damage, preventing its misty escape and destroying the vampire?