Oh dear lord, I just know that if I ran a Vecna adventure, my players would be clamouring for the head.
Practice your evil grin. Roll dice randomly behind the screen. Make notes at random times. If the dice rolls say a PC dies, they die. Have quest-givers lie, cheat, steal, or be misinformed - But only sometimes. Have most play straight, but not necessarily have the whole story, and/or have complications. Practice your evil grin. It is *Never* "just rats". Even if it is "just rats". At least once, have a loved one either be replaced by, or
be, an evil shapeshifter. Make the quest reward something that gets them in more trouble - Either something illegal, that someone powerful wants, that is something powerful but generally useless itself, or simply hard to carry. Make it literal poison at least once. Practice your evil grin. If the dice rolls say a PC dies, they die. Sometimes, it is just rats, except sometimes the "rats" are in an NPCs brain. Breather episodes are fine. Even two in a row. Maybe even three. Occasionally, make something that looks like a breather episode. Then introduce the shapeshifter, disguised werewolves, secret vampire, and/or hidden demonic cult. A stay at an inn can be a great place to have a thief steal their belongings, and it introduces a plot hook. Make sure to make all the rolls and notes behind your screen, just like all the other times you did it when it was meaningless. Helms of Opposite Alignment, Werewolfism, and Vampirism, in certain ways, can all be the same plot point.
Practice this enough, and you can forget about their desire for the "head of Vecna"; they will examine a straight-forward offer of gold for any of a dozen potential hidden meanings.
They may also find this more fun, if they are the type to want a challenge.