To expand on what Scoed said, the game does not consider any sim to be half-anything. Your half alien is apparently a human, so even if they've inherited odd colored skin, the game just sees a human. Your half vampire is apparently a vampire, it doesn't matter that one parent was human, they are a vampire. You can only be one occult form, and spellcaster is an occult form, so that's why the vampire can't learn. If you cure the vampire, they'll be able to learn spellcasting.