Ok, this is a game breaking bug. I have experienced this non-stop on both my saved game and my girlfriends. We have deleted and re-installed the Dawnguard DLC but nothing works. This bug crashes my Xbox360 100% of the time.

Location: Dimhollow Crpyt
Race: Dark Elf
Level: 19
Faction: Joined the Imperial Legion
Follower: Lydia
Quest: Awakening: Find what the Vampires are looking for

Setting: There is a crypt room with 4 passages leading away from it. Each passage has a gate blocking it, the gates are opened by a chain that is pulled. 3 of the passages dead end while the last leads down a corridor into a larger room. Traveling through this passage brings you to a larger room. This room has waterfall to the right and a blooded vampire can be seen in the distance. A doorway is on the far wall. When entering the room, skeletons rise from the shallow lagoon in the middle of the room while the vampire charges forward to attack. When the vampire becomes engaged and comes after me, the game seems to crash when he uses his siphon life ability. I have tested just pulling the skeletons and this is no problem. The issue seems to happen when the blooded vampire uses one of his specials. Using archery I can hit him with my arrows from a distance. This causes the vampire to walk out of sight as he looks for me. If he resets and chills out, when his patrol route brings him back around to where I can hit him with another arrow, he is completely healed.

This is before you get to Dimhollow Cavern.

This needs to be fixed. I would even accept a workaround.

Thank you!

Try deleting Dawnguard from your hard drive (and Skyrim - if you have it installed) and clearing the Xbox cache.