I have just completed the dawnguard MQ with my nord werewolf and i want to join the vamps with my imperial assassin. Is there a way to just go join them or do i have to do the first couple dawnguard quests before i can meet harkon? Oh and dont worry about spoilers

I wondered the same thing. I finished the vampire mq but I got in after I solved that puzzle to release Sarena when I was sent to that dungeon.

thats how i figured it would start. i wish i could join the vamps without even talking to the dawnguard

So i'm guessing you figured it out?

I believe he did.