ThatZenoGuy
Residential Zealous Evolved Nano Organism
Yeah, you can beat the main quest even after killing Vivec. It's balls-out hard, though. Since Vivec is supposed to give you Wraithguard during the main quest, you're without Wraithguard when you kill him. Instead, you loot a "unique dwemer artifact" from his body, which has the model of a left-handed daedric gauntlet. You have to talk to Yagrum Bagarn and get two books from 6th House bases so he can jury-rig a functional Wraithguard from that. Thing is that the jury-rigged Wraithguard deals you 200-250 damage when you first put it on, so you need to be prepared for that. Oh, and while that damage is supposed to be temporal, you actually lose that health permanently in most versions of Morrowind, so you're screwed really hard.
Never played Morrowind, but do you need that gauntlet to win?