I do the vampire thing to go to the Soulcairn, then get cured afterwards...I then wander around with Serana in tow doing various quests and just enjoying things.
Halfway through one of the DG mainquests, I get a message saying "You get a strange thirst when the sun goes down..."
Mmmm...check 'active effects', and sure enough, whateveritisis vampiris....
Now, the only person I've been around for any length of time is Serana...no one else. The only npc I know of which is a vamp, is Serana, and I'm the sort of dude that casts detect dead around people so I can sort out who's who in the zoo.
Dear sweet little Serana seems to have some extracurricular nightly activities, apparently....I'll be checking my pockets and inventory next.