As you all know, it's impossible to join the Volkihar without first joining the Dawnguard, regardless of choice and even if you're a vampire (apperently these "vampire hunters" are too stupid to realize this). Bethesda could have written in a way to join the Volkihar first, but they chose not to. Some of you were wondering if it's possible to just skip this all and head to Dimhollow Crypt, find Serana on your own, and join the Volkihar without ever joining the vampire hunters. This is just an answer to that question:

I entered Dimhollow cave and was attacked by wolves in place of deathhounds and vampires. When trying to enter the next room, I noticed thete was a massivd cave in and I couldn't progress.

So, summing it up, it is impossible to bypass joining the dawnguard by any means, or enter Dimhollow Crypt without joining them, because the path is randomly blocked off. On a side note, you CAN bypass the level ten "requirement" to join the dawnguard before they start recruiting by finding their fort yourself (it's unmarked on the map). Interpret and discuss this as you will.

Cool discovery. But I prefer to think of that as only an initiation to the Dawnguard. I failed when I became a Vampire Lord.

It's still madness to me that they make you help them without giving you any choice. So the Volkihar will take a prospective vampire hunter stupid enough to waltz in with the leader's daughter? And they seem fine with letting random npcs join as "spies" why not you? Here's an easy way to make it make sense:

Vampire hear's about the castle, finds it thinking he'll get in. Volkihar cast a spell on you that makes you black out. Wake up in torture room being interrogated. You say you want to join. They laugh and send you on some suicide mission. You come back and they're impressed. The story goes on from there.

OR

Remember when you go on the quests to turn people into vamp spies? If you ask the questgiver why this perspn was chosen, he'll say it has to do with their blood and the "stars". So basically they think some people are born worthy of joining? Maybe they find out you're on this spiritual list?

See? I made two mediocre solutions in less than 5 minutes. I'm sure the devs could have though of a better one in the time they had.