After you get to Dimhollow Crypt, enter the dungeon and go down the tunnel a bit, then listen in on the conversation between the vampires in the larger chamber. Interrupt their party and kill them, search the cave for some loot. Also find the body of Vigilant Tolan, who said he would meet you here to help you, but now lays dead.

In a small room to the south climb some stairs and pull a chain to unlock a gate on the other side of the cavern, then go northeast through the door that just opened. Enter a big cave with Vampire Fledgling and Skeletons. Then go to the northwest and pull lever to the right of the door to unlock it,  enter another room with some coffins and an Arcane Enchanter, then proceed to the narrow tunnel in the north where the vampires are fighting a Draugr.

Activate another chain to your west and head through the caged passage. Cross the water west and take care of the skeleton and vampire on the far edge of the cave. Climb up, go through another tunnel looting the urn at the edge, then turn to the southwest, fighting your way through several Death Hounds who guard the corridor. The corridor forks into the southeast.

There is a portcullis which requires a lever to open, the lever is to the left of the door. Activate lever, then fight the Wounded Frostbite Spider on the other side, watch out for its poison attacks.

Go south and there is a door leading you to Dimhollow Cavern.