The Beast Immunity mod, removes the 100% disease resistance from Werewolfs and Vampires. I also made a mod where the Sanguinare Vampiris disease (which you gain vampirism from) is an enchantment so that it bypasses the disease resistance, haven't released it yet though. I do know that using the Werewolf's Beast Form breaks the Vampire's feeding ability, so you will eventually get stuck at stage 4. This seems to be a problem with the game itself, a conflict between the two race scripts maybe.