As cited earlier, DMG, page 74.
"Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury type poision, a monk's stunning, and injury type disease. Damage reduction does not negate touch attacks, energy damage dealt along with an attack (such as fire damage from a fire elemental), or energy drains. Nor does it affect poisons or diseases delivered by inhalation ingestion or contact."
Vorpal wouldn't fall into the categories of "touch attack, energy damage, or energy drain. It's not a contact poison or disease.
Greg