I want to know when your about to go into soul cairn on dawnguard side you cant since your not dead so serana asks do you want to be a vamp if you say yes do get the vampire lord form are you a weaker vamp and are still on the daawnguard side

Once finished that quest the Dawnguard won't speak to you when you return, unless you get Falion to cure you. You can still speak with Dexion and receive the next quest. That's as far as I've gotten

so im guessing you cant do the side quests or can you and thanks for the help

Nope not until you've been cured, unfortunately

thank the gods I decided to stay a werewolf and the dawnguard side because I want to get a dwarven crossbow