I would think that Molag Bal would be a little offended that the person went to Clavicus Vile first for power rather than him, I wasn't saying he would be mad because he has a feud with CV because he doesn't. I think a Cyrodiilic pureblooded vampire would be extremely powerful.
Yeah thats what I think as well personally.
Cyrodil vampire:Molag bal i gave you 1000 souls please make me a pureblood!
Molag bal:Why should I? You went to vile go ask him!
Cyrodil vampire:D; but bal pleasee....
Molag bal:I can't hear you! lalalalalala seriously go to vile you like him so much your lifestyle is thanks to vile not me! VILE!
Cyrodil vampire:OK OK i will make it 2000 souls!
Molag bal: *Turns* ok then you got a deal send those souls my way!

I just think cyrodil vampires have to go the extra mile than someone who just follows bal exclusively well thats how I see it.