The first time I met Vivec, I was like, oh its another character that I am required to keep around or else doom the world, so I saved before I tried to kill him just for the fun of it. After a long hard fought battle, I found that the game did not bring up those words. YOU CAN KILL VIVEC AND STILL WIN THE GAME (do main quest line, winning is impossible). In fact I found out you can skip a lot of stuff and finish the main quest line in minutes.