I'll go ahead and restate the important points/aspects of this new vorpal. Hopefully, it'll help in deciding what kind of damage it should do:
1) +5 market price modifier
2) can damage creatures immune to critical hits
3) is force damage, so no resistances/immunities apply
4) uses base weapon damage to determine vorpal damage
That's the basics. Although the vorpal enhancement will improve in functionallity on weapons with high crit multipliers, this is no different than the burst effects, so it really shouldn't make that much of a difference.
So, a choice needs to be made. What kind of damage multiplier will this new Vorpal enhancement use?
1) x4 (probably too low)
2) x6
3) x8
4) x10 (probably too high, but I went ahead and included it because it was mentioned earlier)