I just edited my previous post, should cover this more clearly I think. Please read it.

Just in case, I shall re-re-reiterate.

The OP is talking about making a Vorpal weapon THREATEN A DECAPITATION ON ROLLS OTHER THAN A NATURAL 20. THIS IS A FORM OF CRITICAL THREAT RANGE WITH A SPECIFIC EXTRA EFFECT.

PLEASE READ VORPAL SO YOU
KNOW WHAT WE ARE TALKING ABOUT