I'd say roll damage for the crit as normal, and if there is any in excess of the target being beheaded's full HP, the remaining damage cleaves through as normal.
The vorpal effect is more than powerful enough on its own and doesn't need any boosting from the "Cleaving through foes" variant rule which is exclusively for weaker foes who can't weather a character's normal damage.In this case you have completely negated the vorpal effect. I think it would be better to behead the first target and deal normal crit damage to the next.
The vorpal effect is more than powerful enough on its own and doesn't need any boosting from the "Cleaving through foes" variant rule which is exclusively for weaker foes who can't weather a character's normal damage.
I'm not trying to boost the vorpal effect. I'm pointing out that if you assign enough damage to the first foe to kill it anyway, there is no vorpal effect, only normal damage that kills that foe. You've taken away the cool thing that a vorpal sword does (which is, bypass the dealing damage part).
If the scene is between a high enough level character that he got a vorpal weapon in one of my games and a horde of low h.p minion types, I don't even know if there would be an initiative roll. I would let them describe how they wade through a courtyard of these creatures to get to the real fight behind the big double doors or at the top of the tower.
At a certain point in D&D it is like Superman vs a local biker gang, all you need to see is a panel or two with bikes being thrown around and a pile of guys knocked out bleeding, then turn the page and he is dealing with the real threat who was hiding amongst the bikers.