So, I've had the idea about being a (normal) vampire and a werewolf at the same time. Of course I would tweak everything so the result wouldn't be overpowered, because that wouldn't be fun for me. Luckily, that's possible with the mods I'm using. And also, I'm of course using a mod that allows me to be both at the same time.

I'm using the following Vampire/Werewolf-related mods:

http://www.nexusmods.com/skyrim/mods/9717/?

http://www.nexusmods.com/skyrim/mods/59463/?

http://www.nexusmods.com/skyrim/mods/25392/?

http://www.nexusmods.com/skyrim/mods/37658/?

So I've started a new character and contracted the disease to turn into a vampire, using the option provided by the Better Vampires mod. After 3 days passed, I finally became a full-fledged vampire. So far, so good. But when I now want to turn into a werewolf as well, using the option provided by the Moonlight Tales Essentials mod, my vampirism gets removed, even though it shouldn't due to the Tainted Blood mod. I don't know if that's because of how Moonlight Tales works and if it would work just fine if I would do it the normal way with the companions quests and stuff, or if there is another issue.

Any ideas?

You're using the latest version of Better Vampires right? On the mod page

"Better Vampires 6.1 & 6.2, 6.31 looks to have some compatibility issues at the moment."

So it could be the latest version of Better Vampires causing it.