Everyone is assuming the Paladin struck with the blade.
A character can strike with the pommel of a sword to go for subdual damage.
In old-school D&D, subdual damage was basically you tracking all of the damage you normally would have done, but once the enemy reaches 0 they just yeild instead of die.
To RP this out.. the Paladin would be using the Vorpal Sword. They could clearly cut the head off the opponent if they wanted to.
But, instead, the Paladin is doing subdual attacks...
Think of movies when two people are sparring, and one person is clearly showing dominance over the other, but isn't out-right slaughtering them. The Paladin would be swinging the blade and slapping the opponent with the flat-side of the blade. Or they're side-stepping and letting the opponent fall to the ground.
In this situation, all the "Damage" being caused is just ego damage that is wearing down the opponent's morale.
On the final blow, the Paladin chooses to stop the sword an inch from the opponent's neck and tells them to yield. Or, they knock the opponent out with the butt / pommel of the sword.
Unless the actual blade edge of the vorpal sword connects, then no cutting is taking place, and thus no head-severing is taking place.. at all.
And on a roll of a 20... that is the CHARACTER rolling a 20, not a weapon. So, the character (and thus the player) has say in what they're doing.
If they want to spare an opponent, they can.
But, this kind of subdual damage only works on intelligent foes without god levels of morale. EG: you can't do subdual damage on, say, a zombie. Or skeleton. Both of which (old-school D&D) have a morale of 12.. so they're unshakeable and just keep coming at you until you destroy them. But, on a Vampire you might. On a dragon you might. Any type of foe that knows it's own mortality, and could tell if it's being beaten and can realize the opponent is offering them a way out other then death.
Unless the Vorpal Sword is also an Intelligent Sword that has sway over the owner and thus can force their will upon the user.. I would simply RP this as a character that has a massively destructive weapon and chooses to use it in alternate fashion.
IE: I would role-play this instead of rules-lawyering it.. b/c the rules for the Vorpal Sword will tell you a 20 is a natural beheading.. but that's only if the person holding the sword was going for a death blow.