I'm not entirely certain that vorpal blades need nerfing, but I haven't really seen them in action before. Have they been nerfed historically (that is, in other Games of Death)?
Hm... Although, I know I've been looking at some really insane critical threat ranges over the past few days. Tell you what, here's some proposals, y'all let me know what you think.
a) Natural crit only. The effects of keen, improved critical and other methods of improving the critical threat range for a weapon do not affect its likelihood of decapitating. Only a natural roll of the original, natural, unmodified critical threat range has the possibility of decapitation.
b) Fortitude save vs decapitation. On a successful Fortitude save, take an extra 2d6 damage (based roughly on the +5 cost of vorpal) but not decapitated.
c) Fortification Collar. This is an armored neck collar that prevents critical hits to the neck. It's only real use is against vorpal weapons (and critical hits from garrottes), but some gladiators consider it an indispensable item. It takes up the necklace/amulate location, and comes in three varieties, Light (800 GP), Moderate (3,000 GP) and Heavy (7,000 GP). Light prevents neck criticals 25% of the time; Moderate prevents them 75% of the time; and Heavy prevents them 100% of the time. If NO OHER ARMOR is being worn, they also provide +1 enhancement bonus to AC. For purposes of special abilities, wearing a fortification collar is not considered to be wearing armor. Caster Level: 13th; Prerequisites: Craft Magic Arms and Armor, limited wish or miracle.
I won't do (a) and (b) together - that's a bit much. I may allow (c) regardless, if enough people want them.