If a target survives beheading/it does not occur, vorpal sword deals additional damage instead.
Such a creature instead takes an extra 6d8 slashing damage from the hit.
On a crit, we are to double the damage dice.
Because the beheading only occurs on nat20, and it also triggers a crit, does this mean that vorpal sword deals additional 12d8 slashing damage in total? (if the crit is not prevented)