Vivec should return to a future Elder Scroll game and fight alongside the PC or at least be a quest giver. I wanna see him fight with his spear and skills. This guy is so badass and powerful he managed to banish Azura on his own. Maybe hes hiding in a unknown outer realm or maybe he joined the Psjicc Order. As the only surviving Tribunal i think it would be great to meet him again. If Dagoth Ur is actually still alive himself and has become one with red mountain than it would be even more epic. The epic clash between two old god rivals.

Either he died a mortals death and lives on through CHIM, exists outside of the world through CHIM (I'm not exactly sure how this works) or went on a fun expedition with the Nerevarine.