So everyone is landing on that if the Vampire is hit with Moonbeam, it's reverted to humanoid form and dies?
Couldn't you rule that the effects of the spell do render it back from Misty form to humanoid, but unconscious? The whole point of the vampire turning into mist and then retreating back to it's coffin would be, IMO, to regenerate it's humanoid body. The effects of Moonbeam would force the reversion back to humanoid and essentially start the clock running or because it is outside the coffin maybe it can't regenerate (because it's not in it's coffin) and cannot move(because 0 HP)?
So then the party would make with the head cutting and wooden staking or magic cirlcing or sunbathing etc.
I've always viewed misty escape as more of an uncontrolled action to go into mist and instictly head back home with no actual thought on the vampires part.
From the early discussion I think most people sided worn the vampire going to human and then because he was at 0 hit points would revert back to mist... But since he can't he would die.
I don't think this follows traditional vampire lore at all. I mean, obviously, a DM could rule that the vampire at 0 hp when hit by Moonbeam dies, but that is neither RAW nor in most of the stories told about vampires. Vampires are supposed to be pretty difficult to kill at night or underground. Their near "un-killability" in most circumstances is what makes them upper tier undead, like liches and mummy lords. A second level spell should not be able to finish one off for eternity UNLESS it is done while the 0 hp vampire is trying to rest in their coffin. It makes more sense that Misty Escape IS their true form after they reach 0 hp. Since they can take no actions at that point, they have no way to turn to a humanoid or wolf or bat form from there. This is not like a vampire with hit points left voluntarily assuming gaseous form.