Depending on form(shapeless or non humanoid, say a horse) an the reversion to previous form from damage (6d8x2) or decapitation nothing says that a Vorpal weapon (20) critical is am instant kill for a polymorphed character, some have sited a Jeremy Crawford tweet yet I cannot find that specificity. A DM can do anything but I was hoping to find a Jeremy Crawford opinion. If you can please link here.
It seems fairly straightforward to me. This is my own take on running through the logic.
If a creature (character, npc etc) is polymorphed into a form that has a head:
An attack from a vorpal sword with an attack dice roll of 20, will decapitate the creature's polymorphed form, killing it.
As polymorph states, "The transformation lasts for the duration, or until the target drops to 0 hit points or dies." - the creature hasn't been reduced to zero hit points (allowing death saves) they are just dead. The polymorph spell expires, but they are still dead. Even if the original form didn't have a head, the creature is already dead before the polymorph wears off. Dead is still dead.
If a creature (character, npc etc) is polymorphed into a form that does NOT have a head:
An attack from a vorpal sword with an attack dice roll of 20, will cause an additional 6d8 damage. There is no possibility of insta-kill from decapitation.
In this case, the polymorph spell may expire due to the creature dropping to 0 hit points, which triggers the rules in that spell of returning to it's original form with "the number of hit points it had before it transformed."
Depending on form(shapeless or non humanoid, say a horse) an the reversion to previous form from damage (6d8x2) or decapitation nothing says that a Vorpal weapon (20) critical is am instant kill for a polymorphed character, some have sited a Jeremy Crawford tweet yet I cannot find that specificity. A DM can do anything but I was hoping to find a Jeremy Crawford opinion. If you can please link here.
Ninja
I can't find a Jeremy Crawford ruling, but I agree the polymorph creature is dead if it's true form can't survive without a head.
It seems fairly straightforward to me. This is my own take on running through the logic.
If a creature (character, npc etc) is polymorphed into a form that has a head:
An attack from a vorpal sword with an attack dice roll of 20, will decapitate the creature's polymorphed form, killing it.
As polymorph states, "The transformation lasts for the duration, or until the target drops to 0 hit points or dies." - the creature hasn't been reduced to zero hit points (allowing death saves) they are just dead. The polymorph spell expires, but they are still dead. Even if the original form didn't have a head, the creature is already dead before the polymorph wears off. Dead is still dead.
If a creature (character, npc etc) is polymorphed into a form that does NOT have a head:
An attack from a vorpal sword with an attack dice roll of 20, will cause an additional 6d8 damage. There is no possibility of insta-kill from decapitation.
In this case, the polymorph spell may expire due to the creature dropping to 0 hit points, which triggers the rules in that spell of returning to it's original form with "the number of hit points it had before it transformed."
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There's no Sage Advice that specifically mentions vorpal weapons, but a similar case for Power Word: Kill and Wild Shape is discussed here.