I found a way to become a vampire lord and still be in the dawnguard and smith crossbows and crossbolts. Ok what you need to do is to join the dawngaurd and when he ask's you to go get 2 people after finding serana and after that you go to the girl you found and help her get all the crossbow and crossbolt schematics
after you do all those you should beable to smith them all then you just complete the main dawnguard quest and after that you talk to serana and ask if you can become a vampire and she will make you a vampire lord and when you go into the dawnguard they wont attack you you just cant do anymore missions for them untill you are cured but you can still you their forge to make the crossbows.

Well, I wasn't aware of this.