The exact method by which an individual is converted isn't expressed, so this is a question for SweetCinderella. I personally don't think Demons should be able to be turned into Vampires. I'm not for sure it the change is physical first or metaphysical. Does the soul become tainted first, then the body, or the other way around? In any case, demons essentially are as tainted and corrupted as they come in this roleplay. They're notably powerful, probably more so than Vampires. I don't see how you could corrupt them further or make a physiological change to them via an inferior strand. 

Now, what we might be able to do, is create a demon bloodline that could be the predecessors to vampires, ie ones that started as demons and 'infected' mortals somehow. That again would require Cinderella. I may entirely be wrong and it may be possible; I just think such would be odd. It would be like giving an angel faerie wings. Aside from the Aesthetic, why? Or, how?

If you're wanting the act of being 'turned' while having a demon, I just find that entirely odd. Maybe a half-demon, half-mortal that was converted, but I personally don't see a full-blooded demon being turned over. Cinderella is the final call here, of course, but my opinion is that the idea itself has a slight, intrinsic flaw.