I've only had my vampire sim get sick once, which I thought was strange but honestly it didn't really bother me. It probably shouldn't happen, but then again I also feel vampires should be less immune to some things or at least players should have options. For instance I kind of wish vampires were more affected by temperatures, not dying or getting sick by it, but at least uncomfortable and changing their outfits to suit it. My vampire is rather precious and prefers a nice home with central heating than a cold musty castle lol, but I always have to change his outfits manually. I know others would disagree though. There are so many different variations on vampires after all.