At the moment if you meet Gandrel (the hunter in the bog) before meeting Astarion PC doesn't seem to connect the dots, even though Gandrel tells the vampire's name.
This is also not adressed in the journal, if you meet Astarion after the Gandrel encounter either.
In case you don't recruit the Astarion and arrive to the hunter, but do not tell him Astarion's location I got a pop-up dialogue on Lae'zel and she behaves as if he was already recruited.Screenshot
If you talk to him after this in the camp, you can tell him you met the hunter etc., but he will not really have an option to bite yet.
I assume these chains might be changed anyways.