I am getting to this slowly. I don't critique prestige classes very often so I'm not speedy.
This could be written more clearly. Again. Don't use BAB for HD. Circumstance bonuses stack and are much better than untyped bonuses.Blade of Infinite Edge(Ex): For rounds equal to her base attack bonus per day a Vorpal Warrior can grant any slashing weapon that she wields the vorpal quality. If a weapon that she wields during this effect has the vorpal quality from another source she can decapitate on the next highest number. These rounds can be started or stopped as a free action but must be used in minimum of one round increments.
How is this?
Blade of Infinite Edge (Ex): For a number of rounds equal to her HD, a vorpal warrior grants any slashing weapon that she wields the vorpal quality. If any slashing weapon she wields already has the vorpal quality from another source, she gains a +1 circumstance bonus to decapitate her opponent. She can add or remove the vorpal ability as a free action, but the ability itself must be used in minimum of one round increments.