I cant speak for dragonborn as i havent completed it yet.
But i did dawnguard as soon as i could, because i was making a necromage vampire. Youll want to finish dawnguards main story asap once you start it though. I only had about two vampire attacks that happened and easily stopped both of them. Also all of the loot i got from dawnguard is leveling with my character --- GT: Combat 00
So after doing some research it seems that none of the gear in Dawnguard is levelled but certain items in Dragonborn you have to be level 60 to get the best version. Can anyone confirm/deny this? --- "Dang girl, if you weren't a figment of my imagination, I'd wanna have your baby."
I think right around level 8 vampires show up and start attacking towns. So.. right around then would be good to start it. You'll want to finish it once you get ti started though unless you really, really like having a follower you can't really get rid of. --- GT: HiPOJaK I'm so happy I could just tear out your intestines and strangle you with them! -Sheogorath