Mmm.. Yeah, I've always avoided putting a vorpral weapon into the game. One random roll and what should have been a challenging fight is over in an instant.
Check out this new article Wizards of the Coast posted recently:
Ask Wizards: 07/06/2007
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?