Okay so im a dawnguard member and I cant get in castle volkihar, but what if you're a vampire? Can you just walk innfor dawnguard and wreck havoc?

Well, I walked right in there at the beginning...The guy even said something along the lines that they're defenses were so poor at the moment that a Vampire could just walk right in...From there, picked up the quest to get in good with the vamps. And yes I was a vampire from the beginning.

Did you get special dialogue because you were already a Vampire or did Harkon not mention it?

...Nothing special except Serana mentioned she knew what I was. The Dawnguard were none the wiser.
Apparently Glowy eyes are not suspicious.

I know right?

I thought people would have noticed you having red and black eyes but they didn't..

I also thought people would have noticed your glowing gold eyes and a very gaunt face..but no.

Interesting... Our good faction is oblivious as Oblivion. (Not hell, hell does not fit into ES Lore )

Heck I even picked up a complete set of Dawnguard armor during the Vampire line. Like, who needs 'em? Best of both worlds baby, now kiss the ring, mortals!