The only reason I didn't say that increases fro 19 to 20 is that it depends on whether the vorpal warrior can already do that. There are feats and abilities you can have from other classes that may already allow this. Don't want to give the class an ability it can't use because it already has it. Increasing the threat range by 1 is better since it stacks. However I should have specifically said to decapitate a foe.