I still think my wording for Force Wave Slash is clearer, but the addition of Force damage (with accompanying save for half) is a nice touch which I approve of. So it would look like 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 may decapitate her opponent upon critical confirmation with a natural roll of 19 or 20. 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.
Force Wave Slash (Su): A Vorpal Warrior may take a penalty on attack rolls to increase the reach of any slashing melee weapon she wields. She gains +5ft reach / -1 penalty, to a maximum of a -5 penalty for +25ft reach. The damage her weapon normally inflicts becomes force damage instead of slashing damage. A successful Fortitude save (DC is 10 + 1/2 the vorpal warrior's HD + Cha modifier) halves the damage.