If you are not a vampire you need to either let Serana turn you into a vampire, become a vampire on your own or let her trap a part of your soul. What are the implications of these choices? Do you need to get cured of vampirism before progressing with the Dawnguard? Does your soul ever get returned to you?
You can recove your soul after accomplishing your objective in the Soul Cairn, Valerica will give you the quest, it is fairly trivial.
Should you become a vampire, you will need to be cured in order to continue working with the Dawnguard, you'll get the quest to get cured from members of the Dawnguard.
Interestingly enough, getting your soul back will net you 1 black soul gem, while getting cured of vampirism will cost you 1 full black soul gem.