You do not have to kill Serana to complete the Dawnguard questline on the vampire side.
If you join the vampires, Harkon will bite you and you'll be a vampire lord, even if you were only an ordinary vampire before. However, you don't have to have the vampire lord power to progress with the vampire side of the questline -- you just have to be a vampire. You must be a vampire to receive services or quests from the Volkihar, but not to speak with Serana and have her follow you (or for any quests where you're only interacting with her or Valerica). Conversely, you cannot be a vampire to receive quests and services from Dawnguard members (after "A New Order").
Serana can make you a vampire during "Chasing Echoes," although you'll receive the vampire lord power. Any vampire (but not some named vampire NPCs) can give you the vampire disease, but they must attack you with their vampire drain spell (the red crackling spell). If they infect you with Sanguinare Vampiris, you just need to wait three days and you'll turn into a vampire.
Vampires can be found at all http://en.uesp.net/wiki/Category:Skyrim-Places-Vampire_Lairs locations and also as part of some random encounters in the wilderness.