Well, like I said, if you feel this version's enhancement is too powerful, restrict the death save to only creatures that rely on heads, and to those who are only 1 size category larger than the wielder of the vorpal weapon, unless special cases permit for creatures larger than one size category over the wielder.
Or, you could restrict sharpness and vorpal like you do with the current vorpal: the enhancement only takes effect on natural crits, not crits with bonuses to crits factored in (keen, improved crit, etc.). At least, I read SOMEWHERE that the current vorpal requires natural crits to work
The current vorpal is powerful enough, since it can technically lop off the heads of giants, dragons, etc. on a natural crit. This version weakens it slightly (by giving SOME chance at survival) but makes it a little more broad in scope, with dishing out dmg to things immune to crits and such. So what.
I am guessing, however, that the force damage from sharpness and vorpal does not have str bonus/enhancement bonus factored in, etc., as that WOULD make it too powerful to be +5. With the force damage just being damage die however, I think it is fine as +5.
And yes you could compare flaming or flaming burst weapons and their enhancement modifiers and such, but just remember, those weapons do something on every hit. These two only make your crits nasty rather than making every hit hurt more.