In every incarnation, the vorpal jabberwock is a more powerful version of the regular jabberwock, appearing as J. Like the regular jabberwock, it is especially vulnerable to being beheaded by Vorpal Blade (and other methods of decapitation, depending on the variant).
The vorpal jabberwock possesses multiple beheading attacks, each with a 5% chance of instantly beheading you on every successful hit (100% chance if you are polymorphed into a jabberwock or vorpal jabberwock yourself; 0% if you are currently headless or amorphous). Additionally, jabberwocks can grow up into vorpal jabberwocks.
In UnNetHack, the vorpal jabberwock behaves very similarly to GruntHack's version, except that the chance of beheading is only 2.5% per hit, and canceling the vorpal jabberwock prevents it from beheading anything.
In EvilHack, the vorpal jabberwock behaves very similarly to GruntHack's version. Additionally, it is one of the alternate forms the Wizard of Yendor may take on, and monsters may use a wish for a figurine of a vorpal jabberwock.
The SLASH'EM version of the vorpal jabberwock possesses no beheading attack, will not grow up from a regular jabberwock, does not generate in Gehennom, and is not especially vulnerable to being decapitated by Vorpal Blade.
SpliceHack's vorpal jabberwock acts mostly like SLASH'EM's version, except that Vorpal Blade will still always decapitate it.
"Beware the Jabberwock, my son!
The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
The frumious Bandersnatch!"
He took his vorpal sword in hand;
Long time the manxome foe he sought --
So rested he by the Tumtum tree,
And stood awhile in thought.
And, as in uffish thought he stood,
The Jabberwock, with eyes of flame,
Came whiffling through the tulgey wood,
And burbled as it came!
One, two! One, two! And through and through
The vorpal blade went snicker-snack!
He left it dead, and with its head
He went galumphing back.