Funny that you had Vecna wanting to be the God of Death in your game given that he has never had an interest in that (Orcus is the one who wants to)
From Open Grave, p 212:
Two of Vecna’s chief rivals are the Raven Queen and Orcus. Both claim dominion over death, though each, including Vecna, has a different vision of what the afterlife should be. Vecna believes that creatures should serve him in both life and death.
And from the RC, p 53:
Vecna is the evil god of undead, necromancy, and secrets. He rules that which is not meant to be known and that which people wish to keep secret. Evil spellcasters and conspirators pay him homage. He commands them to . . . Oppose the followers of all other deities so that Vecna alone can rule the world.
I'm not a canon junkie by any means, but using the basic descriptions of NPCs, gods etc can make record-keeping easier! In this case, Vecna is said to oppose all other deities, to seek to rule the world (both living and dead) and to rule that which is not meant to be known (which, in 4e, includes the ultimate fate of the dead).
Lets not forget that Vecna managed to get one over on the Lady of Pain and partially rewrote reality before his avatar was destroyed. (Even then he still came up on top as he gained the power of a lesser god out of it when he was a Demigod before his plan.)
This sounds like material from some module that I don't know or use, and that is not part of the core 4e cosmology.
Vecna has always been a schemer who's interest was never to take another gods portfolio, but instead to destory all other deities and powers so he would be the sole remaining god with power over all reality.
The only difference I see between (i) "destroying all other deities", including the Raven Queen, to gain "power over all reality", including death and the dead, and (ii) taking over the Raven Queen's portfolio, is that (ii) is only part of the way to (i).
i think Vecna aiming to replace the God of Death is a much too low of a goal for him.
It seems to me that a necessary condition of ruling both the living and the dead is to rule the dead; and that a necessary condition of ruling the dead is to replace the Raven Queen.
He should be turning her into his puppet with some devastating secret or weakness and have her attack other gods while he waits in the shadows scooping up the losers divine essences.
No one's stopping you using that idea in your campaign!
In my own campaign, he was attempting to learn the Raven Queen's name (which is a secret).
The first scheme, hatched by a priest of his, was a little convoluted, but involved (i) learning the Raven Queen's name, either (a) by prising it from the beings from beyond the stars with whom she had hidden it, or (b) recovering her body and then having that body touched by a strange man who had the power to learn the name of any corpse that he touched, so that (ii) the name could be traded to Orcus in return for access to certain tombs that hid a secret map to Torog's Soul Abattoir, so that (iii) the Soul Abattoir could be diverted to power-up Vecna rather than Torog, so that (iv) once Orcus struck the Raven Queen, using the power of her name, Vecna could then strike Orcus and take control of both death and undeath.
This scheme came unstuck when the PCs themselves took the secret map, killed the cultists who had access tothe beings from beyond the stars, and then killed the priest (and killed him again in lich form).
Once
the PCs killed the exarch of the Raven Queen who had done the deal with the creature's from beyond the stars, Vecna sent an angel of secrets to acquire her name directly, but the PCs stopped that too.
Then, when
the PCs destroyed Torog's Soul Abattoir, Vecna took advantage of the fact that one of the PCs had the Eye of Vecna implanted in his imp familiar to try and divert the power of those souls to Vecna, but the PC in question thwarted this also. And he subsequently was able to
sever the connection between Vecna and his Eye.
Most recently, the same PC communed with Vecna again to learn a secret way into Orcus's palace on Thanatos - using, as the focus, the body of
an Aspect of Orcus which had been magically bound, after being defeated, by drawing upon Vecna's power. The PC ended up
stealing the secret from Vecna (secretly!) so as to avoid having to do a deal.
Overall, Vecna's star has not been ascendant. Which tends to happen to recurring enemies of the PCs in a long-running campaign. But his methods and motivations (manipulation of secrets; necromancy; the desire for power at the expense of other gods) have been pretty clear and consistent.