Let us start from scratch here....
Vorpal states that on a roll of a natural 20 you decapitate your opponent as long as you confirm the critical threat. This is what Vorpal does....
Blade of Infinite Edge grants an already Vorpal weapon the chance to decapitate on a natural roll of 19 or 20 pending confirmation of the critical (an ability I have NEVER seen in all my years of brews and 3rd party shiz). Heads Will Roll further increases this to any natural roll of 18, 19 or 20 causing decapitation with confirmation of the critical.
It has nothing to do with reach or anything to do with threatened squares. The 'threat range' we are referring to is the critical threat range, more specifically an increase to the 'vorpal threat range'.Threatening an opponent: you threaten anyone who is adjacent to you. There is no "range" to this....<snip>
It also has nothing to do with, nor does it bare any resemblance to, what you're saying here:
Pardon me, but this makes no sense in the first place since Vorpal has nothing specifically to do with attacks of opportunity, it is purely about the number you roll on the die when you attack with any attack.....even if you mean to simply give them a better chance of confirming criticals in very specific circumstances, then that's just a REALLY bad version of 3.5's Power Critical (dunno if it even exists in PF), which is a feat granting +4 to confirm any critical threat with any one weapon you have focus for....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.
Basically I think you're entirely missing the intention of the OP and therefore not understanding what the ability is meant to be doing.