Guys you're just trying to break the game.
Critical hits get weapon damage maximized, thus you calculate the highest damage you could roll with your attack, then take it as fixed damage. no roll. You can call that a damage roll or whatever you want, but the maximized damage does not include a die roll, it's, well, fixed (so no vorpal effect on it). Then you roll any EXTRA criticall damage (wich is effected by vorpal).
Now what? You give a vorpal weapon to a minion monster, then since minions always do fixed damage, the minion makes infinite damage with a vorpal just becouse you don't roll any dice, but you call it a damage roll anyway?
please, so lame.