Vivec's voice is Legion.
His Sermons are truth after the Dragon Break, which is the only timeline we now know. It is mixed with a share of metaphor and does not take place entirely in the corporeal realm, but we cannot directly dismiss it without going back to knowing nothing.
The battle of Red Mountain is sermon 36 and there is nothing after it. "Vivec and Nerevar God-Adventurers" took place a good while before. Or in an un-time after, which hardly counts in your linear, all-or-nothing view.
Believe it or not, there was a reality outside Vivec. The world and all its mysteries do not exist purely at Vivec's whim, nor does Vivec understand everything. He is a warrior-
poet. He is not a god. Eternal life (which he never truly had; more augmented than eternal) does not define godhood. The Dwarves used the tools of Kagrenac along with Dagoth Ur and Sotha Sil, but none of them ended up as gods. Honestly, the Dwemer ascended to skin and Dagoth Ur turned into diet-Cthulhu. As for the Tribunal, they have histories, birth places, and many, many, oh so many faults.
Vivec's voice is Legion? What is that? Milton?
I think Vivec had the potential to become an actual god if the Heart of Lorkhan was tuned to the right frequency, but even then, he'd probably end up as Sep or yet another face of Akatosh, heralding a Dragon Break to end all Dragon Breaks.
In short, he's a warrior-poet, like Ulysses. He has a corporeal body while his mind floats out of time. I'd compare him to Shor, but I don't think Vivec was bound to Nirn in the same way but more like Vivec chose to stay on Nirn out of a crazy mixture of egoism, that is the prospects of being a god among mortals and the nationalistic and ethnocentric goals of his mortal past which he can now shape.