"Fae'run Vampires" and their Thralls might be different from our real world folklore.
There are many things that do not make much sense to me in general.
Why the no mirror image thing? Why the fear for a christian cross?
Or maybe even silver, or garlic, etc.
Vampires really are just "homebrew" creatures to some degree.
But yeah the fact that Astarion is nowwhere classified as Undead feels like lazyness to me.
