When you are a vampire, that is trying to escape your vampire master, but also you are going to be turned into a Mind Flayer... its just fucking confusing. Also vampires in D&D are undead, not blade vampires, how is it that they can have their non-living bodies transformed into mind flayers isnt that like transforming a pile of dirt or unliving meat?
More on topic (kind of), there exist vampiric illithids and afaik their origin is a mystery. Perhaps it's just that, a vampire (spawn) undergone ceremorphosis.
I mean, perhaps Astarion wouldn't turn into a mindflayer if this was normal ceremorphosis. Perhaps
if the netherese magic preventing the tadpole from turning us into mindflayers was removed
Astarion would just die. That'd be a pretty brutal ending for him...Larian?