Attacking a save is a weird mechanic and it throws off when actions take place. Making an attack takes a standard round so how does this fit in? I don't like giving a PC an extra action.
The bonus that is gained is pretty hefty. Would you allow a PC to have a +8 bonus to make a Reflex save as part of this Prestige Class? Granted I am VERY conservative when it comes to such thing but granting but this effectively means that a PC is making a save that includes her BAB in addition to her Str modifier which is probably higher than her Dex modifier.
Furthermore, this mechanic resembles the PC who announced, "I attack the darkness." This will no doubt lead to derision in some places when a character makes an attack to avoid the Reflex save to avoid catching fire. Just don't go there.
Mind over Body means you make a Concentration check rather than a Fortitude save. A concentration check uses a similar mechanic to the save so it's not too dissimilar. A concentration check is your skill modifier and your ability modifier. It doesn't also add a BAB bonus.
I am NOT of fan of using BAB in ways it was not intended. If you had wanted this to be a Strength check instead of a Reflex save that would be a lot more fair.
Cut The Air (Ex): Instead of making a Reflex save, a vorpal warrior can choose to make Strength check as long as she is holding a slashing weapon and use the result instead. If the check is successful, the vorpal warrior avoids the effect as if she had evasion.