I have just finished the dawnguard questline for a second time and this time on the vampire side. I was just wondering, what happens to the dawnguard now?

After the dawnguard questline I expected to at some point attack fort dawnguard on the vamp side but no... on that note the dawnguard played pretty much no part in the vampire side.

Can I go back and just casually mingle with them or what?

there is a sidequest for the vampires where you kill everyone in fort dawnguard,just keep asking if the vamps need help until you get it.

ok thanks