Vampires in D&D can be at least neutral - another Faerun vampire elf, Jander Sunstar, is a good example. Astarion obviously was an evil person as a mortal and 200 years of torture unsurprisingly didn't change him. Someone in this thread mentioned this already but
Considering how many non-linear small things this game offers, I bet that both redemption and staying evil will be possible for him. His background story is so specific it must be.
Personally I believe that his character is much more than just a manipulative monster as many people may see him.
I think you may be right to some extent. Someone on Reddit noticed that