I don't think Astarion will ever let them become one ('Trust me, it doesn't happen' line comes to mind). Even if Tav drinks his blood ingame, it doesn't change anything. It seems like he also has to willingly let them go while being drained for it to work. In the epilogue you can have a discussion with him about freedom and he gets annoyed about it. He fully intends to keep Tav by his side forever.
Yeah I don't want that myself, I just thought it might happen by accident if Tav drinks Astarion's blood during lovemaking. I, on the other hand, wouldn't do that so as not to offend Astarion So it doesn't work that way. There's basically no "unfreedom" in it, just that the title is "spawn" instead of "vampire". But nobody calls Tav a "spawn" in the game, and even if they did, she will be able to stand up for herself. By the way, I tried biting Oblodra myself (I downloaded it after Astarion killed her and tested it) - nothing, no negative effect, obviously, actions within the gameplay do not affect the story in any way.
Originally Posted by Ametris
I adore that line and the way he says it. The whole time he's trying to persuade Tav to join him, he's clearly manipulating them. That's why his "I love you" doesn't sound very sincere because he is not expressing his affection but is forcing himself to say it to convince Tav. It seems like he feels awkward articulating real feelings and especially doing it right after ascending (when he feels almighty) puts him in a vulnerable position. The simple "thank you" to me is the real love confession. That's the moment where he feels 100% secure and calm and the scene is very intimate. There is no more need to put on a show and use fancy words. His feelings are actually very intense, just look at the way he watches Tav while drinking their blood just before thanking them!
Yes, I specifically left a save before this scene and periodically revisit it! Just like the scene of the first night, and my favorite scene from the second act, after which I went like a fool with a smile on my face for a couple of days... This has never happened to me in any game, there have been interesting romances, there were "favorites" that I always carried around with me and laden with the best artifacts, but to be like Astarion, this is the first time. Maybe because of the living face and the fact that he looks like a real person, or his character and his story, I don't know. Games are really reaching some new level right now.
Originally Posted by Ametris
I did it to tease him, since he loves power so much. He looks adorable when he refuses her the second time and thanks you afterwards.
So it doesn't work that way. There's basically no "unfreedom" in it, just that the title is "spawn" instead of "vampire". But nobody calls Tav a "spawn" in the game, and even if they did, she will be able to stand up for herself. 
