Sorry but I LOVE the tiefling/vampire purring headcanons so much like I HAVE to gush about it !!!
I feel like Astarion would definitely already know that tieflings could purr when they're very happy/content/in love. So the first time he gets Tanit (my tiefling Tav) to truly purr deeply, with her whole chest, he is SO proud of himself it's almost insufferable. Those warm, bubbly feelings in his own chest are a LIL more intense than expected, but it's only because his nice simple plan is going so perfectly. Of course he's happy.
BUT I'm willing to bet NEITHER of them knew about the fact that vampires can purr because it's simply not very common for vampires. They only ever do it when they feel exceptionally content and safe, usually among another member of their kind that they form a rare bond with. Such happy circumstances are rare for vampires and Astarion has had a shitty life even by vampires spawn standards so of course neither he or any of his siblings ever got to experience such a thing.
And so one night they're cuddling, maybe after Cazador is dead and Astarion finally has that threat no longer looming over him. He feels amazing. He feels more loved and safe and happy than he can ever remember feeling. Tanit starts raking her fingers through hair. He instinctively lets out a sigh of contentment, but instead what comes out is a...low rumbling??
They're both lying chest-to-chest so for a moment he thinks maybe it came from her, but no. This purring is much deeper. The vibrations very clearly came from his chest this time. They both look at each other and Astarion is maybe a little confused and embarrassed and possibly wondering if he's somehow involuntarily picked up her mannerisms. Until they both learn it's perfectly natural for vampires to purr as well. Just another thing they share in common.
And then they proceeded to spend every cuddle session thereafter purring together like two very happy big cats.
