Ok, so maybe a brief explanatory note should be made, introducing a Vorpal Range...? The idea is a simple one, but wording is definitely a bit awkward. That said, I don't recall coming across ANYTHING else which has increased the chance of vorpal beheading so maybe the clarification is unnecessary. I imagine a large part of the issue is the general ignorance amongst many players and even a few DMs as to how the vorpal enchantment works. A simple enough solution might be to create a Greater Vorpal enchantment, purely for the purpose of this class, and detailing that perhaps?