The key here is that it specifically mentions two things... one, that this happens on a roll of a natural 20, and that it also requires a confirmation for the critical hit. The thing about rolling the natural 20 does not specify that this is a function of being a critical hit though... in fact, 3.5 went specifically out of its way to make vorpal NOT part of being a critical hit, but something like a nat 20 only "super critical" hit.3.5 SRD said:Vorpal: This potent and feared ability allows the weapon to sever the heads of those it strikes. Upon a roll of natural 20 (followed by a successful roll to confirm the critical hit), the weapon severs the opponent’s head (if it has one) from its body. Some creatures, such as many aberrations and all oozes, have no heads. Others, such as golems and undead creatures other than vampires, are not affected by the loss of their heads. Most other creatures, however, die when their heads are cut off.
The Vorpal ability requires you to confirm the critical.SRD-Fortification said:Fortification: This suit of armor or shield produces a magical force that protects vital areas of the wearer more effectively. When a critical hit or sneak attack is scored on the wearer, there is a chance that the critical hit or sneak attack is negated and damage is instead rolled normally.
But the fortification ability does not make one immune to a critical hit.FEADIN said:The fact that some creatures can be beheaded by the attack is not linked to their immunity to criticals, they are immune but can be beheaded (no extra damage), the fortification negates, it's different no?
Nail said:...what ever happened to "Swords of Sharpness"?