Vorpal is a weapon enHancement not enCHantment. It's just one of my pet peeves.
Threatening an opponent: you threaten anyone who is adjacent to you. There is no "range" to this. See here:
I do not recommend that you mess with this as it will likely end in frustration for other players if the vorpal warrior can undermine an action that they were going to take. Also, it makes it very hard to envision. How do you attack someone 10 feet away when you have a reach of 5 feet? It's not like you suddenly become a stretchy Mr. Fantastic and can extend your arm that far. Now if you want to give the vorpal warrior THAT ability, you could, but it should probably be a separate ability.Threatened Squares
You threaten all squares into which you can make a melee attack, even when it is not your action. Generally, that means everything in all squares adjacent to your space (including diagonally). An enemy that takes certain actions while in a threatened square provokes an attack of opportunity from you. If youíre unarmed, you donít normally threaten any squares and thus canít make attacks of opportunity.
Is this better?
Heads Will Roll (Ex): Whenever you threaten an adjacent opponent, you gain a +1 Circumstance bonus to any attack of opportunity you make to sever your opponent's head as long as you are wielding a vorpal weapon.
Remember that Circumstance bonuses stack with all other bonuses (including other circumstance bonuses as long as they aren't from the same source), so this makes the most sense. I am not a fan of untyped bonuses. Most of the time, failure to add a type of bonus is sheer laziness on the part of the creator.