f you side with the http://www.uesp.net/wiki/Skyrim:Dawnguard_%28faction%29 you can convince Serana to cure her vampirism after you kill her http://www.uesp.net/wiki/Skyrim:Lord_Harkon and finish the main questline. This is dependent on the conversations you have with her during the questline. A conversation option "Have you always been a Vampire?" will become available after Serana brings the http://www.uesp.net/wiki/Skyrim:Elder_Scroll_%28Sun%29 to http://www.uesp.net/wiki/Skyrim:Fort_Dawnguard and you ask her to follow you. There are a few possible choices you can make while conversing with her about this however, if you ask her if she has ever thought about getting cured she will refer to her condition as a gift and state that she will never seek a cure for it. Should she say this you can still ask her to seek a cure after her father has been defeated but she will refuse and then refuse to speak to you about it further should you ask again. If you do wish for her to cure her Vampirism after the main questline then choose the option "Are you all right?" instead of asking her about a cure. Once you ask this she will end the conversation but you will still be able to convince her to seek a cure at Fort Dawnguard after her father is dead.
If Serana chooses to become human she will no longer be able to provide the player with http://www.uesp.net/wiki/Skyrim:Bloodcursed_Elven_Arrow nor will she be able to turn the player into a http://www.uesp.net/wiki/Skyrim:Vampire_Lord.