Okay, so I've played the dlc six- ways- to- Sunday, and I read somewhere about the vampires crushing the Dawnguard at their castle....so how does this start? What are the prerequisites? Help.

It's a random radiant quest. Just talk to the vampires in the castle and ask what you can do to help. Eventually you will be given the quest.

Thank you. One more thing; I would imagine this side quest needs to be done before the final vampire fight?