Except that I'm not quite a vampire. It's very bizarre. I gain the Champion of Night and Nightstalker's Footsteps abilities, but not Vampire's Sight. I gain the weakness to fire and sunlight, but I don't gain the immunities to poison and disease. Additionally, my appearance doesn't change (my eyes remain normal) and nobody can recognize me as a vampire at any stage. Not the Dawnguard members. Not even Serana (as I still retain the option to ask her to transform me into a vampire). I can't even pick up the cure quest from Falion because as far as anyone is concerned, I'm not really a vampire. The only way out of it (apart from reloading) is to ask Serana to turn me into a vampire (even though I already kind of am) which she'll do. At this point I'm recognized as a vampire (and my eyes go yellow as they should) and I can then pick up the quest to cure myself.
It's a bug, obviously, but it's too bad that it isn't bugged the other way around (gaining the immunities but not the weaknesses). If that were the case, I'd just stay that way and be a "Daywalker."
Anyway, has anyone else experienced this?
Edit: I should also add that these occurrences seem to be indefensible. I was wearing a Necklace of Disease Immunity the first time this happened and it did nothing, probably because sanguinare vampiris doesn't even factor into this situation. It's just... boom... suddenly you're a quasi-vampire. Furthermore, it doesn't seem to matter how far away from Serana I happen to be. It appears as though as long as we're occupying the same cell, she can infect me somehow.