Q: Dear Sage
Since a vorpal weapons needs a successful confirmation roll to confirm a critical before its attack lops off someone’s head, can the successful use of the fortification armor ability stop a vorpal weapon critical from taking off a head?
A: (This answer replaces an earlier, incorrect answer. What can we say: vorpal is a tricky weapon property, and the Sage spoke a little too quickly when he said that fortification trumps the snicker-snack of a vorpal weapon. The Sage apologizes for the confusion and hopes this sets the record straight.)
A vorpal weapon’s trademark effect is triggered on a special kind of critical—on the roll of a 20 (and a confirmed critical) it cuts off a head. Creatures without a head are, of course, immune to this effect, but the description calls out a number of creatures immune to critical hits, but that have heads, as subject to the weapon’s effect. Furthermore, it states that while these creatures may be subject to a vorpal weapon’s decapitation ability, the decapitation may not kill or destroy them. These statements create an exception to the normal critical hit rules in a way similar to (though not as clear as) some of the weapon burst powers featured in Magic Item Compendium.
Because of these exceptions, being immune to a critical or having an ability that negates a critical has no effect on the vorpal’s head-taking properties. Only the lack of a head makes you immune to the vorpal’s decapitation, and only in the cases where a head is not necessary for the creature to be alive (or it does not destroy creatures that are not alive) can a creature survive its decapitation effect.