Well now, filthy swits, this good man here is asking us how do we prefer to pronounce his name. I am in the habit of saying Vivec, but occasionally I will say Vehk.
In general I think it's safe to assume that unless stated otherwise, whatever pronunciation the voice actors use is correct, so I pronounce it as the voice actors do.
And yes, I always refer to him as Vivec, it's the name he is labeled with in-game, and Vehk only seems to show up in some in-game texts, the usual name NPCs use is Vivec, so I go with that.