He unified Tamriel under a single rule, and because of this Tamriel was able to repel Akavir when it invaded Skyrim. He managed to do what no other man, elf, beast, daedric prince, or god had ever done before, and was to conquer the entire continant.
He was able to do this with the help of the Numidium a tool that can help achieve asention to godhood (see the king of Worms in Daggerfall).
Vivec was a very weak God. Even the Nerevarine could defeat him.
The Nerevarine was not a God. It's quite possible that Tiber Septim was simply decent at stuff.
When the Nerevarine 'killed' (its not canon, and future lore from 5th era says Vivec is still alive) Vivec had lost most of his power and was in a very weak state. At the highest of his power he could turn back Daedric Princes.
He was powerful when Tiber faced him down, essentially a god at that point in time.
Talos is not a mere mortal that is pretending ot be a god, he really is a god.