Once you pick a side during Bloodline i pretty sure your locked to that side. You can pick the Dawnguard finish the questline then ask Serana to turn you in to a Vampire Lord but it sounds a bit late for that.
If you side with the Volkihar Vampire Clan, members of the Dawnguard will seek you out and try to destroy you. They are more than likely sporting crossbows, though you probably will not come across enhanced or Dwarven versions (and definately will not be able to get the exploding bolts). I think the best you can get is the standard crossbow with the Dwarven bolts looted from Dwarven Spheres. I recommend that you do not join the vampire side to become a Vampire Lord unless you cure yourself immediately after getting the achievement because you do not regenerate Health, Magicka, or Stamina in sunlight. Also, being a werewolf is better because you can go to Solsteim if you have the Dragonborn DLC and get rings that augment your abilities in beast form (more damage per hit but take more damage from enemies, slow time when in beast form, health regenerates in beast form).
Hi, old topic but there is a way, I want to share, go data files in skyrim launcher and uncheck dawnguard.esm, load your last save, now all dawnguard DLC content is gone now. Save game and quit, check dawnguard.esm, restart the game and load your last save. Now go any city, Durak will approach you and Dawnguard quest starts from beggining. Dont forget you may lose some content
But game may crash when you uncheck dawnguard.esm and you may never be able to run the game without dawnguard DLC. There is still a way but you should use a little piracy. First backup your skyrim folder in your documents (İt may not necessary but pirate version of the game may corrupt your orginal game data on your documantary folder), you should download skyrim, dragonborn DLC and last updates, install them into another target folder (dont replace with steam version) and run the game, load your last save, now all dawnguard DLC content is gone again. Save game and quit. Delete all pirate content from your PC, copy and paste your backup skyrim folder into your documantaries (dont forget to protect your last save). Now launch skyrim from steam and load your last save. Go any city, Durak will approach you and Dawnguard quest starts from beggining.
For some reason, after going to the soul cairn I was doing volkihar quests despite having sided with the dawnguard first. I chose to become a vampire to go in so this may have had something to do with it, but 8 got cured by falcon first thing upon coming back.