I think the explanation(in a WOJ I think)is that magic and life are linked which is why Wizards live so long, but because vampirism is the stealing of life force (directly or through blood) it dampens any magical ability the vampire has. From Harry to Snakeboy drop in power perhaps. Thomas if rid of his demon might hit a significant wizard power level. Whether that is any use to Harry in curing Thomas is another matter, it does set Thomas apart from other Whamps. Thomas certainly seems to exercise his limited talent, so it hasn’t withered like Charity’s.
Harry treats Thomas as the world treats Harry, as a thug, I need to read that story again, myself, it’s been some time and probably worth going through the earlier short fiction again following the most recent stories, we got a couple of really substantial nuggets in The Law and major revelations in PT/BG, that may alter those stories in retrospect. Probably worth re-reading the original comics again for that matter as they are canon. For example in Peace Talks in the flashbacks Malcolm was teaching Harry slight of hand and close in magic. In Wild Card Harry beats Puck in a card game by apparent sheer luck, but I now wonder, did Harry use slight of hand to cheat because he realised Puck was only looking for magical and not mundane methods of cheating.
A re-read is required.
Harry’s got plenty of luck but all of it is bad.
Thats why other characters POV stories are useful, Harry is an unreliable narrator. He portrays Marcone very nearly right, for example, but is way off on Toot in contrast.