To be honest I'll have to make a whole new character, one with evil tendencies, to join the Vampires, and I generally don't play evil. Out of the 10 or so characters I've retired thus far only one was evil, he joined the DB and did all the daedric quests, but that's generally all he did and I needed someone to do that so I could see those quests. Generally my characters are 'good guys'.
I'm glad your opinion differs from mine. What a boring place this world would be if it didn't.
Sadly, I'm going to make a hard save and run through both sides. The one I enjoy the most will become my main save. I think I'll enjoy the vampire side more though. The trailer made it seem like the story was mostly written for that side.