I am a member of the Dawnguard who was formerly a werewolf, but I recently had Serana turn me into a vampire. I had already mastered the werewolf perk tree, so if I decide to become a werewolf again, will I still have all the perks? Also, I had already completed the Dawnguard's faction questline and was planning to remain a vampire hunter and use my vampiric powers to destroy evil vampires, so will Dawnguard members still give me assignments even if I'm a vampire?

the perks stay. At least....they did for me.

Yes. Same with the Vampire Lords. Whatever perks you unlock you keep.

No. The Dawnguard will tell you to get cured before giving you work.