Is there any way to start the awakening part of the Dawnguard quest line without joining the Dawnguard? My character is a vampire already so I don't see why a vampire would join vampire hunters.

There isn't a way possible you have to go through a portion of the dawnguard. It doesn't take long though. But it makes since city's aren't going to spread word on how to join someone who wants to feed on them so at least it makes sense.

I am pretty sure you can't even take the boat to Castle Volkihar until you start the Dawnguard side. Remember they said the quests converge then diverge.

Ive done the quest line before but I wanted to do it for my vampire character I just dont see why in need to join the Dawnguard to start Awakening. A vampire wouldn't join vampire hunters.

...The Dawnguard can't tell a Vampire from a Troll. There's no penalty incurred for being a vampire when you first go and talk to them.

If it's role play constriction that are rustling your jimmies for whatever reason, I just did it under the guise of sizing up these so called Hunters in case I needed to deal with them.