So I know there are a lot that will be like me, I was vampire all the way but almost decided Dawnguard last minute cause Im a lvl 66 who really hasnt focused on Magika, I was liking the crossbow and Dawnguard seemed more me. So if you have trouble deciding as I did, how about you do what I did so you can do both on your main?
-When you get to the vampire castle, save an extra saved slot
-Now when you're ready to do the other story line switch over
Simple, hope it helps.

Also, you can cure and reaquire VL form a many times a you want. Even from the dawnguard side.

Aela also gives 1 second chance to become a werewolf again.