Cut The Air seems rather pointless. How do you judge the attack against a save? You need a mechanic for this. I recommend removing this and letting the vw save normally or give her a +1 competence bonus to Reflex saves if she is holding a slashing weapon.
Cut The Air (Ex): A vorpal warrior gains a +1 competence bonus to Reflex saves as long as she is holding a slashing weapon.
It seems rather clear to me. you simply make an attack roll in place of a save and compare it to the dc, a la the mind over body maneuver.
Why do you consider it pointless? It is significanly stronger than your rewrite, and it fits the concept of the vorpal warrior better, IMO.
My Homebrew (until I get enough that I make an extended sig)