I only watched the Blade movies once each, so I barely remember anything. I guess you could have some stylish action, but that's kinda too shallow...? Inuyasha could be interesting, I guess, but youkai!=demons, really, and there's the time difference too...
Rosario - As much as I like Nero, I kinda find myself warming up to the idea of Vergil at Youkai Academy, if only because his take on all the characters would be so different from usual. I really can't see him starting up a harem (intentionally at least, I can see him having a huge fanclub). I mean, Moka's powerful, but her outer personality is about totally opposite what Vergil would respect. (Ninja edit: Tsukune turning into a ghoul. <- Would Vergil approve? I mean, making your lover stronger, crossed with what happened to Eva, could be interesting.) Kurumu would totally want to hit that, and after getting totally scorned, would get more serious. (I always kinda liked how she went from shallow flirt to much more caring and dedicated.) <- Obviously, I'm okay with breaking up Tsukune's harem, since he's totally ending up with Moka anyway. Then however you want to handle Yukari (/Eva being a witch) and Ruby's "humans are asshats!" Mizore... well, I got nothing there
I guess you could have a very quick time-pace. I mean, you could skip weeks at a time, to keep the story moving quickly.
I honestly think you'd have to do the school time first, and then after graduation, Vergil goes and runs off, and of course his
harem friends former classmates go to look for him. Too bad he already jumped into Hell.... not that I can see this stopping characters from a story like Rosario+Vampire. They'd probably it's all a good idea to go after him. (And drag Dante along, haha. He would totally complain about his brother knowing all these hot chicks... and Tsukune.)