I'm assuming you mean Sunder (as in Sunder and Keening...)
. If so, it is quite a problem as you can kill Vivec to bypass a certain main plot part, needing Sunder isn't it. I'm almost positive you can't finish the game without it... so all I can suggest is that you use the console thingy to cheat yourself in a new Sunder, although never having used cheats I don't know how you'd go about that
.