I think the vampire blood switches the cells to fast healing, but he is capable of using external magic if he REALLY needed to. He was still able to use his own internal magic without affecting the bloods healing effect. If he pulled power from around him, his body would go back to the standard healing speed.
In College Arcane his injuries were well on the way to being healed by the time Tanya gave her blood, so he was healed almost immediately & could use his magic later without affecting the process. I don’t think they knew about it at the time though, since the theory only came to light from CAECO’s mom’s research in DD.
I can’t seem to find my post on vampire witches, but I don’t see why one wouldn’t have been turned sometime over the last few thousand years. Vampires & witches are closely associated in the books, so it’s almost a certainty. What happens afterward is anyone’s guess. My thought was, if one was turned & retained their magic, it might grow like other vampire powers & an old witch/vamp would be a major magical power. Maybe one of the old ones Senka won’t talk about.