The only way this will work is if you assume Vivec is not a god without the heart and even then it's very doubtful that any could. The Nerevarine was immensly powerful even without any aid, so he or she managed to kill two gods, but that wouldn't work in a mod where Vivec is assassinated. A Tribunal member might be able too, but Almalexia already killed Sotha Sil and we know for a fact Vivec was still alive by that time.
An insanely powerful mage like Divayth Fyr might have a chance, but that's far from sure because even without the Heart Vivec would be incredibly powerful, your best bet would be some kind of order of assassins working together, or perhaps a Daedric prince who somehow managed to get to Tamriel with their full power.
The problem with Divayth Fyr is why would he even try? I really can't see him holding a sufficient grudge against any of the tribunal to even bother.
Also, what stops Vivec from doing that never really being dead trick he mentions in his dialogue? Is this mod going to take place solely after the MQ?
As for who could, I'd imagine that any of the four corners could give it a good go, as well as the PC. I doubt the run of the mill telvanni wizard would have much luck, and a group of very powerful assassins might have a chance.