It would probably make more sense if the tadpole (or the plot) changed Astarion to a Dhampyr, which is a living creature with vampiric traits, not a true undead. Illithids don't usually try implanting tadpoles into undead, for the obvious reason of: it doesn't work (shockingly few things are viable targets for tadpole implantation, actually. Gnomes, for example, usually aren't). We've got special magic tadpoles though, so now he's alive again, with lingering vampiric traits.

The last time people tried combining vampirism with Illithids, it... didn't go well.