## The vampire is destroyed

The combat rules on p. 197, *PH* say:

> Most DMs have a monster die **the instant it drops to 0 hit points**, rather than having it fall unconscious and make death saving throws.

The [sunburst][1] spell's effect is instantaneous, but so is the damage. There is no time delay given in the rules between an attack or spell hitting or causing a saving throw, and the resulting damage. With no defined difference in timing, it all must happen in the same instant. 

That means the vampire is in sunlight at the instant where it takes damage, and cannot transform to mist but instead is destroyed, if the damage reduces it to 0 hit points.

Because the rules also do not *explicitly* say that there is no time delay, you might want to check with your DM in advance if they rule this way if you are unsure about how they would handle it.



 [1]: https://www.dndbeyond.com/spells/sunburst