it stays 6d8 and the rules as written, it wouldn't be 12d8 it would only be 7d8.

DM Discration and also what rules they use, it is Small to Large Creature with heads that get it shortend, a creature with no heads or are Huge or bigger will gain the damage as said, tho is it a creature with two heads and with in the parameter of a decapitation, then normaly you only slice one heaqd of unless The DM say otherwise.

Plasmoids are immune to this... 

Most likely, with rules as written,it either kills them or you get a crit. So double damage if not dead ig.

Quote from Jimrabbit >>

When you attack a creature that has at least one head...

In any other context, the question "does this creature have a head" would be the most idiotic thing ever said around a table. But in DnD... 

 Um...Jellyfish.

its an extra 6d8 on top of normal crit damage

Quote from CapitanCJ >>

its an extra 6d8 on top of normal crit damage

 That is not what the PHB says. ALL damage dice on the attack are doubled for a crit. That includes the extra damage, any smites added on, any Hunter's Mark, any Hex, etc.

stays at 6d8

What do you guys think is the most powerful magical weapon in Ur opinion 

I think this might depend on the character class as some weapons in the hands of a Wizard might be useless. As a weapon, the Vorpal sword is pretty good, especially in the hands of someone like a Fighter that makes multiple attacks and thus increases the odds of utilizing its devastating feature.

As much as I initially felt that the extra 6d8 is static bonus to damage, I do believe it is also doubled. Rolling a 20 on the d20, and not necessarily rolling a critical hit is what triggers the additional slashing damage (provided they have no head to cut, etc.). So for a greatsword, a roll of 20 yields 2d6 + 6d8 + STR. Then the effects of that natural 20 also being a critical hit take affect, becoming: 4d6 + 12d8 + STR. 

Dumb, basic sword

This sword destroys campaigns so i love it

This sword destroys campaigns so I love it 

Most likely, the 6d8 would become 12d8, because there are cases when a 20 does not result in a critical hit. For example, a target wearing Adamantine armor would not suffer from a critical (see Adamantine Armor). 

Does it go snicker-snack?

My favorite weapon! This is OP with divine smite.

It’s +6d8, not +12 d8 on a nat 20. Also, common sense. Isn’t +12d8 already op? (That is my nat 20 attack, so yes I am OP.)

Huh? Snicker-snack?

N-N-N-N-N-NO! It’s only 6d8, not 12 d8! Or else it would say so!

weapon dice doubled

6d8 added (undoubled) as a result of the crit.

Longsword example: 2(1d8)+3+6d8