Raphel event is triggered after the first time you finish a potential lead. among them is nettie, gut, the gith guys. It has no relation to Cazador. you can have this conversation with Astarion before even learning he is a vampire. The (kinda) simple solution is to change his dialogue if you didn't discover he is a vampire yet. of course, that would require writing and extra recording and shit. another one to the pile I guess.
