So why the Head of Vecna?
Well, if you are familiar with the aesop / Morality Play of the Head of Vecna, I am guessing your question then is why specifically did the opposing group choose the Head of Vecna as their lure for their con? And why on Oerth would a band of adventurers chop off their head to get it?
Well there is a reason for that.
In the old Greyhawk campaign world there was an ancient lich who became a God (of secrets) named Vecna.
From his Wikipedia entry:
Originally from the World of Greyhawk campaign setting, Vecna was described as a powerful wizard who became a lich. He was eventually destroyed, and his left hand and left eye were the only parts of his body to survive. Even after he achieved godhood — being a member of the third edition's default pantheon of D&D gods (the pantheon of Oerth)—he is still described as missing both his left eye and left hand. Vecna's holy symbol is an eye in the palm of a left hand.
What is important to note is the bit about his hand and eye. They were left behind (by Kas - who ended up in Ravenloft via the plane of Ash), after he was destroyed and were imbued with "great power". So much that they became artifacts. This goes all the way back to the Eldritch Wizardry book, so this is "in game" Lore from the very beginning of D&D.
And if you were to find them and wield them, you had to lop off your hand or pluck out your eye. (BTW: You nasty, Vecna with your dirty raisin-eye in other people's bleeding eye sockets.)
So surely the players of these adventuring groups searching for this Head of Vecna were well aware of these actual (as in "actually part of the fictional campaign world" actual) artifacts - The Hand and Eye. And that is what made the lure of the Head of Vecna such a perfect con.
Because if the eye is powerful, then the whole head has to be bonkers powerful, right?
And of course you would have to lop off your head to wield the Head of Vecna.
That's how the other bits O' Vecna work, right! (No, not Moander, Vecna)
From that hilarity ensues.
All that said, I wouldn't be so sure that I would not make the same series of errors only to end up in the same place. It can happen - especially if one never goes back and questions one's assumptions vigorously and regularly. And really - who does that in real life, let alone in a D&D game.