So a vorpal sword does 1D8+3+strength bonus slashing damage on a normal hit, is useless against creatures immune to slashing damage, decapitates on a 20 if the criteria are met, and does an extra 6D8 points of damage on a 20 if the criteria is not met.
A Balor's sword does 3D8 slashing +3D8 electrical damage + strength bonus on a normal hit, does electrical damage to creature immunes to slashing damage, and does 18D8+strength bonus on a critical hit.
How again is the Balor's sword the same as a vorpal sword?
The fact that a regular vorpal sword is useless against creatures that are immune to slashing damage is not a meaningful difference, since (as far as I can tell) there is nothing that is immune to slashing damage from a magical sword. Plenty of stuff is immune to slashing damage from
non-magical swords, but nothing is immune to the damage from a vorpal sword, that I know of.
Note also that a vorpal sword does
not have a base damage of 1d8; a vorpal sword can be
any type of sword that deals slashing damage, and the base damage of a huge longsword
is 3d8.
The fact that a regular vorpal sword can take the head off of any chump, but not anyone cool, is not a meaningful difference from the function of a balor's sword which can also not take the head off of anyone cool. The rules simply don't bother to declare an exception for what happens when a balor critically hits a chump, because there's no reason why they would be fighting in the first place, and in those rare situations where it
might come up the outcome is functionally
similar to simply dealing an extra 27 damage beyond normal critical damage.
Even if a Balor's sword
isn't technically a Vorpal Sword - which is a fair argument, though a bit nitpicky - it still operates
as though it was a Vorpal Sword for the purposes of this discussion. Its
actual in-game functionality has
at least as much in common with a Vorpal Sword as it did in AD&D, where it was explicitly
vorpal (adjective, not noun) and also detected evil/good and exploded if anyone else touched it. It doesn't have +3 to hit or damage in 5E, but it probably didn't have +5 to hit or damage in AD&D either. It is a
sword which is
vorpal, even if it's not the item which is named Vorpal Sword.