(Inspired by this question about adamantine armor)
The item description for adamantine armor states:
While you're wearing it, any critical hit against you becomes a normal hit.
The item description for a vorpal sword states:
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 roll of a natural 20 is also a critical hit, which adamantine armor reduces into a regular hit. So, if a creature wearing adamantine armor is hit by such an attack from a vorpal sword, does the adamantine armor also prevent the beheading property of that sword?