Im roleplaying a vampire character who basicaly hates other vampires and see the volkihar clan as rivals and what better way to rid skyrim of his rivals than by joining the dawnguard posing as human and a vampire hunter? my question is can I do this? all npcs in the game dosen't seem to notice the fact that your a vampire but will the dawnguard? I know that serana can turn you into a VL before entering the soul cairn but how will de dawnguard react to this?
Thanks:)

The Dawnguard will give you a vampirism test at some point. I am assuming if you fail it, they will not let you join them. They are vampire hunters after all. It takes a lot of convincing just to allow Serana to hang out there.

You won't be able to the dawnguard quests they will tell you to go and get yourself cured before you can talk to them.

If you sided with the Dawnguard but had Serana turn you into a VL at the end of the DLC, you won't be as powerful of a Vampire Lord as you could be if you joined the Volkihar when you were given the chance. Only the Volkihar will give you the Radiant Quests that you can get the Amulet of Bats/Gargoyle, and Ring of the Beast/Erudite from, as well as the Bloodstone Chalice.