Maybe im wrong, but i bet Astarion tells me that Vampires are made by Vampire drinking someone's blood ...
So ... if that is so simple, you probably should be turning into Vampire if you allow him to drink from you ...
But there is that tadpole thing, maybe that will block all other metamorphosis out. :-/
What i wonder more, is consequences for this ...
Since in the same sentence Astarion tells you that when this happens, the young Vampire is just a puppet for older one.
And he can be freed just by drinking his master's blood ... wich dont happen often, since no Vampire need competition.
And i bet that "no Vampire" part includes Astarion as well. :3
