Board Thread:Lore Discussion/@comment-26440657-20180214090426/@comment-2600:1702:26D0:6CA0:949B:28D2:AB78:B25C-20180613210110

Dawnguard is Canon. Before you go to the Soul Carin, serana asks you if you want to become a vampire so i think the dragonborn did become a vampire in the middle of the quest until the end of the quest where you get cured from falion in the end of the quest.