I like that. So the attacker gets to choose which limb is removed on a failed save, but can only choose "head" if he rolls a 20 on the attack roll. Unfortunately that leads to the situation where you roll a 20, confirm the crit, and the victim still makes his save so you "only" do double damage.
Maybe it should be vorpal on a 1 for the save. That way you combine the "something got cut off" with the vorpal effect in one roll.
Maybe it should be vorpal on a 1 for the save. That way you combine the "something got cut off" with the vorpal effect in one roll.