Wild Shape specifies that:
You automatically revert if you fall unconscious, drop to 0 hit points, or die.
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?