Can you prevent a Vampire from using Misty Escape with the Moonbeam spell?

Question

I'm wondering about what will happen if a vampire is dropped to 0 hit points, transform into mist, then fails the moonbeam saving throw.

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.

While it has 0 hit points in mist form, it can't revert to its vampire form, and it must reach its resting place within 2 hours or be destroyed. Once in its resting place, it reverts to its vampire form. It is then paralyzed until it regains at least 1 hit point. After spending 1 hour in its resting place with 0 hit points, it regains 1 hit point.

The moonbeam description says:

A shapechanger makes its saving throw with disadvantage. If it fails, it also instantly reverts to its original form and can't assume a different form until it leaves the spell's light.

The moonbeam forces the vampire to revert to its original form, but the Misty Escape trait also states it can't revert to its vampire form.

What will happen then? Does the vampire got destroyed? Does the vampire revert to its vampire form with 0 hit points? Does moonbeam fail to forcibly revert the vampire?

Answer 1

In this situation, where there are two conflicting exceptions in play, it's going to be up to the DM to determine how they interact.

Personally, I would interpret that Misty Escape means the vampire is unable to willingly revert to humanoid form, not that it's physically impossible for it to do so; and so I'd rule that in the light of a moonbeam (assuming it fails its save and has no uses of Legendary Resistance remaining to allow him to succeed anyway), the vampire is forced back into humanoid form at zero hit points, and is unable to resume its mist form, so it is destroyed.

However, a DM would be just as correct if they chose to rule that the vampire is physically incapable of returning to humanoid form no matter what moonbeam says, so the spell deals damage (to no effect) and nothing more.

I tend to favor the players doing something cool and clever, but which way a DM rules on something like this may depend on how big a disruption to the game losing that vampire is going to be. If the PCs weren't supposed to be able to kill him outright and the whole point was to start a ticking clock while they frantically search for the resting place, then it's probably to the DM's benefit to rule against the players. If it was a minor monster and defeating him won't make much difference to the story, it's better to give it to the players.

Answer 2

If the vampire was under the effect of the moonbeam when it hit 0hp, it would die due to being able to transform into mist. If it succeeded in achieving mist form, reverting back wouldn't kill it, only drop it to 0hp. All three possibilities you mention are viable and can be used to further the story.

If the DM wants to preserve the final blow, he could say the moonbeam damage is negated due to already being at zero hp but not unconscious. The vampire then reverts to an unconscious and defenseless form, still at 0hp.

If the DM wants to get it over with, he could say the moonbeam reverts the vampire to normal form and takes the spell's damage at zero hp, being destroyed.

If the DM plans for an event where the party chases the vampire to its resting spot, or wants the vampire escape, he could argue that the vampire is incapable of being reverted.