Unfortunately, none of them gave me an answer. Then one of my viewers told me that the first appearance of the word was in a novel called Through the Looking Glass by Lewis Carroll.
He took his vorpal sword in hand,
longtime the manxsome foe he sought
So rested he by the Tum-Tum Tree
And stood awhile in thought.
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.
The viewer also said the sword appears in Dungeons and Dragons as a weapon with a chance to decapitate an enemy.
Actually, Vorpal Sword in Romancing SaGa 2 has a unique sword technique which always critically hits against humans and sometimes kill them instantly.
I guess Akitoshi Kawazu, the producer of the SaGa series, got inspiration for the weapon from D&D. It is well-known among SaGa fans that he would import American board games and play them with his friends in his formative years.
Later, I learned that there were swords with the same name in some Final Fantasy games. I also found out that the swords are called シャープソード (Sharp Sword) in their Japanese version. What a shitty generic name!
Vopal Sword | Final Fantasy Wiki