It seems it is possible. At least it is for me. Just recently my vampire sim was uneducated for the third time and got pregnant for the third time. Both other times it ended up an alien, but this time I appears its a vampire alien. It's only a toddler atm, but the alien babys original form is human skin coloured and normal hair and eyes. The alien design over the crystal that hangs on the sims head is there, and I can still design a disquise and have him wear it, but at the bottom right corner it shows the vampire smiley face? Idk what is going on but it seems it can be possible to have a mix of a vampire and alien.
