It's actually about the cure but the dialogue has alot to do with this. Didn't want to put the spoiler in the title. Though you might want to know anyway.

I started a paladin character. An anti undead warrior. So of course I joined the Dawnguard. I completed the quests slightly troubled by the fact that a vampire is following my every move. But I kept Serana with me because I like her as a follower and I knew about the cure.

I knew Serana's vampirism could be cured because I've seen the 'cure' dialogue with my main [assassin] character [and I might have read about it somewhere]. Then I avoided this option since I was a vampire myself and did not want for her to be cured. But with my paladin character it was kind of a must for me. I can't have a vampire follow me around while I go out killing her kind. So naturally I ask about her cure. Nothing concrete. No cure option yet. Well ok.. maybe later I thought and went on hacking and bashing my way through the quests. Numerous vampires have felt the fire of the Dawnbreaker [Including Serana.. unwillingly I swear].

Finally I finish Dawnguard. Time to wake up Serana. Long story short my character got barked at for asking her nicely and backed down. Apparently it's weird to suggest curing herself if she's going to live with vampire hunters.

This was kind of important for me [my character] so I looked it up. Thats when I found out I messed up in the dialogue. It seems you cannot ask her about curing herself if.. you want to cure her? To make matters even worse I couldn't get rid of her either because of some bugs. So I was forced to walk around with some vampire while I'm out on a holy quest to eradicate all undead from Skyrim.That's kind of messed up. I bet she enjoyed herself too.

Look, I enjoyed Dawnguard. I really did. But this kind of ruins it for people who are trying to role-play, something Skyrim is otherwise perfect for. Don't you agree. Now I know about the console commands. I'm seriously considering it. But I'm kind of a paranoid person. I've read about the problems that could occur when using console commands. So I'd rather not touch it [or is this weird?].

For a last resort I loaded a save from before I asked her about curing herself for the second time in case there's something else I could do to get the right dialogue back. But the question is: Is there?