I want to unlock the vampire skin overlay so I can use it in CAS and maybe try and figure out how to make it available to younger Sims. I changed the flags on all the vampire-related things to end in 00, but it's not showing up anywhere (I checked skins, makeup, costume makeup, clothing, even jewelry...), so I must be missing something. Should I try to clone it instead, or will that make no difference?

You can't find it in CAS because a vampire overlay isn't a skin, makeup, costume makeup, etc. It's an overlay, and it goes over an existing skin. As such, you won't be able to use it for kids or toddlers, because the game just isn't coded that way (unless there's some hack out there that actually allows your kids to become vampires... but I've never heard of such a thing).

There are a few vampire replacement overlays out there. If you really want to see how it's done, I would suggest downloading one of them and having a look at it in SimPE to see how it's put together.

Hmm. Is it possible to clone it (or one of the replacement overlays) and categorize the clone as makeup or something? I know there are custom skin overlays besides vampire defaults that can be applied manually (tattoos, for instance) but I would have to extract the vampire texture somehow before I could even try to give it the tattoo treatment...

...this is starting to sound like an interesting endeavor. Maybe I'll try and work it out after I've done my last assignment for the semester.

I don't see why you couldn't clone it. I've seen the vampire overlay texture in the game files. I've never done anything like tattoos before, but I don't see why you couldn't turn the vampire overlay into a "tattoo" that you could put on whatever skin your Sim was wearing.