You don't have to be a fan of Potter to enjoy the attraction. I'm not a big fan of the Spider-Man character, but I still find that attraction phenomenal. No reason you can't appreciate the thrills and technology of Forbidden Journey, even if you're not a fan of Harry.
I could have done most, if not all, of Universal in a day, when it was off-season, but it would have felt a bit rushed I think. Therefore I imagine doing it via a VIP tour would be quite possible.
They're great parks, and a lot of things are repeatable. I'd suggest two days unless you're really short on time.