This campaign background is every part of awesome:
The Seven Brides of Vecna
You could do it with having them contain the bits of the Rod of Seven Parts, actually. Just make them, effectively, part of a magic item themselves. That is, all on one location they are a power source or magic source beyond their individual utility.
Seems pretty horrible to describe mummified wives of an arch-lich in terms of utility, but in a way, I think that makes Vecna all the more evil. "Hey, I'll get married to seven women, turn them into undead and make them part of an unliving construct that enhances my personal power." If you didn't think he needed to be thwarted before, well, how about now? Plus, it opens up the possibilities beyond "destroy them" or "gather them" to "can you free them?" and "can you undermine Vecna's power by changing his wives?" It also allows for a mix of motives - perhaps some loved him, some wanted to use him and failed, some feared him, others were marriages of convenience for mutual gain. So you won't be able to deal with each the same way.
Even if they aren't part of some magical construct or power circle for Vecna, they can be sources of knowledge about him. Perhaps they know his true name, or each holds a piece of it as part of the marriage ceremony that bound them together. Perhaps being married seven times to brides each possessing part of control of Vecna's magic made for some multiple of his power. Gather them all, learn from them (or decipher the true name from examining their inanimate remains), and you'll either gain power yourself or undermine that of the great arch-lich. It would be a major campaign if your goal was to kick out the power posts of a major NPC in the canon of D&D and destroy him.
(Obligatory Monty Python reference here)