Well, depends on the timing. If it's after his loss of divinity, it was him, if it was before, it was his avatar.
.......no he was himself, the only beings that use Avatars are aedra and Daedra, he wasn't a god on their scale and he was very much vulnerable to mortal follies, like believing a lie from Cyrus and stating the Cyrus would kill them both over simple dishonor...
Daedra make Avatars because they ARE the spheres in Oblivion, when you go to the Deadlands in Oblivion the game, The Deadlands IS Dagon, but I guess you already know that. Vivec doesn't have this aspect of existence he just had alot more freedom. they were all just jacked up on Lorkhan juice.