You are able to kill Vivec anytime in MW. Would killing him anytime after he gives you Wraithguard mean that you (The Nerevarine) mantle him?
Nerevar was a friend to him, but Vehk betrayed and killed him. The Nerevarine could do the same to Vehk. Of course, it's the motives that matter. Also, I assume that mantling one with CHIM does not give the mantler CHIM. That would be quite far-fetched.