I’d consider myself to be an Elder Scrolls fan(that’s the series these two are from, btw, Vivec from ES: III, Morrowind and Dagon from pretty much all the games, particularly ES: IV, Oblivion), and having read the lore surrounding these characters, I can say straight up that this is a ROLFSTOMP in favor of Dagon. The only people even within the games who think Vivec could take Dagon are the deluded idiot Dunmer priests who freaking worship him.

Let’s count the advantages:

GODHOOD:

Mehrunes Dagon was born divine. He is a Daedric Prince; think of them as the Elder Scrolls equivalents of the Chaos Gods from Warhammer, with Mehrunes probably being most similar to Khorne. He rules over his own realm, the Deadlands, as a god.

And Vivec? A mortal dark elf sorcerer who used tools created by one of the most powerful dwarven enchanters to draw a small bit of divine power from the heart of a dead god(Lorkhan), granting himself and three others a small share of said god’s powers.

POWER:

Vivec, along with three others of similar powers to him, were unable to defeat Cyrodiil in its 80-year-long war with Morrowind; the best they could do was hold back the superior armies of a single human empire. In fact, according to Almalexia’s prophecy, they would actually have suffered defeat if there had only been two of them(Vivec and Almalexia) fighting for Morrowind instead of all three(with Sotha Sil there as well).

Mehrunes Dagon, in contrast, is considered a threat to the entire world of Tamriel, to the point where his physical presence on Nirn, with all of his powers, was considered the point where victory was impossible without divine intervention. His armies were also able to wreak havoc across all nations of Tamriel, and possibly the other continents of Nirn, at the same time.

IMMORTALITY:

Mehrunes Dagon is completely immortal. Wait thousands of years and come back? Dagon is still in his prime. Cut off his arm? It grows right back. Destroy his essence entirely? He comes back from the dead in a moment. As a Daedric Prince, he is protected by the very rules of the universe itself, because Daedra cannot die; their immortality is literally more extreme than the Nine Divines themselves. At the end of Oblivion, you can hit Dagon with every magical attack in the game, slice him with every weapon, again and again, and he won’t die. If your character is powerful enough, you can literally reduce him to a puddle of red goop, and he regenerates fully formed in a few moments.

Vivec can’t compete with that. All he has is a divine cheat that prevents him from aging: he can be killed. In Morrowind, if you power up your character enough or use cheats, you can literally overpower Vivec and kill him, and he won’t come back.(Doing so before he fulfills his vital role in the Main Quest is not really a good idea, though.) He is not fully immortal, and can be slain in combat. Furthermore, when he was separated from his source of power, he lost most of his divine powers and began to slowly age again.

CONCLUSION:

It is really no contest. The best Vivec could hope to do is temporarily banish Dagon from the mortal realm before the Daedric Lord killed him. Even if Vivec won, it would be a temporary victory at best: Ol’ Mehrunes could eventually come back and try again. If Vivec lost the immediate battle, it would be game over for good: He’d be dead.

Vivec banishing and defeating Dagon in single combat is not very likely, either. In the backstory to the games, Mehrunes Dagon was able to manifest an avatar(not his real self, but a weaker version to interact with the mortal plane) onto Nirn in a contract with a witch. He effortlessly destroyed the entire city of Mournhold, and then preceded to fight Almalexia and Sotha Sil(the two characters that have the same powers and abilities as Vivec, and get their powers from the same source). It took all of the powers of TWO entities of Vivec’s level to banish this avatar of Dagon back to Oblivion, and the wounds they received were so intense that Almalexia nearly died. Furthermore, they were at the height of their power at the time, and by the time Vivec appears in Morrowind, his powers have been waning for some time, and he is not nearly as strong as they were during their previous fight with Dagon.

Daedric Lord, FTW!