[Faceman]

I think it would be a nicely balanced item if the Vorpal Hit had no save.
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In a GWW the Hackmaster will on average deal 175dmg
5.5+12*10 (0.5 Critical Hit and 0.5 Critical Miss cancel out each others dmg gain/loss)
but it also gives you a 40% chance to die during that GWW. Now that sounds great for a last resort weapon. Much like the Elven Moonblade at the end of the game. If you can't beat 'em any other way you have to risk your life.
EXCEPT: High level warriors (lvl17+) have a Death saving throw of 3 (15% chance of failure). Add the RoP+2 and the Helm of Balduran and the Vorpal is no problem anymore.
If the Vorpal was "no save" however it would be a weapon you keep in your pack (don't know how if it cuts through everything) and only get out if you're incredibly desperate. Should death occur you could always be raised by a cleric.