Vorpal Sword vs. Wildshape

Question

Wild Shape specifies that:

You automatically revert if you fall unconscious, drop to 0 hit points, or die.

and

When you transform, you assume the beast’s hit points and Hit Dice. When you revert to your normal form, you return to the number of hit points you had before you transformed. However, if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. For example, if you take 10 damage in animal form and have only 1 hit point left, you revert and take 9 damage. As long as the excess damage doesn’t reduce your normal form to 0 hit points, you aren’t knocked unconscious.

Vorpal weapon specifies that:

When you attack a creature that has at least one head with this weapon and roll a 20 on the attack roll, you cut off one of the creature's heads. The creature dies if it can't survive without the lost head. A creature is immune to this effect if it is immune to slashing damage, doesn't have or need a head, has legendary actions, or the DM decides that the creature is too big for its head to be cut off with this weapon.

So what exactly happens here? If the Vorpal triggers the "kill the creature"-portion, the creature dies (Power Word: Kill kills Wild shapers through it for example) but RAI feels like the creature should just revert. Thus the question is about whether the Wild Shape form's head is crucial to the Druid: if it is not, the "dies"-part of Vorpal's wording wouldn't trigger and the Druid would instead just take 6d8. I wonder if there are some obscure rules about the importance of body parts in Wild Shape?

Answer 1

The druid dies, then reverts to their normal form

Cutting off the head is not an effect that reduces hit points. Hit points are an abstract quantity (PHB p 196):

Hit points represent a combination of physical and mental durability, the will to live, and luck.

When decapitated, you do not die due to loss of hit points, you die due to losing your head.

The reasons you revert automatically are:

You automatically revert if you fall unconscious, drop to 0 hit points, or die.

Since the beast is not losing hit points or falling unconscious, it would not revert unless it died.

Either the beast can live without a head, in which case the vorpal blade does not kill it, or it cannot, in which case it will die and then revert.

Reverting to druid form here is not due to losing hp, it is because you already died.

Currently none of the creatures in the rules that are typed beast can survive without a head. This means the druid will die, and then revert to their normal form.

The situation is the same as for Power Word Kill, on which the Sage Advice Compendium ruled:

What happens if I’m polymorphed or Wild Shaped into a creature with fewer than 100 hit points and then I’m targeted by power word kill? You die.

Answer 2

You die

At present, no beast can survive without a head; it’s quite a niche talent. The vorpal blade is clear that the creature dies. Therefore, you’re dead. Then you revert to your original form.

Answer 3

A Druid in Wild Shape can survive with a lost head…by reverting to their usual form.

There are two crucial phrases in the Vorpal Sword description:

When you attack a creature that has at least one head with this weapon and roll a 20 on the attack roll, you cut off one of the creature's heads. The creature dies if it can't survive without the lost head. A creature is immune to this effect if it is immune to slashing damage, doesn't have or need a head, has legendary actions, or the DM decides that the creature is too big for its head to be cut off with this weapon.

I’ve bolded the crucial phrases: the description talks about requiring a head to continue living. However, a Druid in Wild Shape can survive if it loses its beast-head! Because a druid would usually just revert to their usual form when reduced to zero hit points, we can’t say that the druid “can’t survive without the lost head”, even if a mundane version of the beast form would not be able to survive. And since a druid in a wild shape who is decapitated would usually just revert form, it doesn’t need the head to survive, and the automatic kill effect does not trigger.