Was the any mention of them pre- DLC? nope, they just pulled a new faction out of nowhere. See, I like the idea of a faction of vampire hunters, but there was already at least two other factions they could have used instead.
The first faction that springs to mind is the http://www.uesp.net/wiki/Skyrim:Vigil_of_Stendarr they were already in skyrim and could have been put to more use then they are already. On the UESP it says that they are " dedicated to wiping out Daedra and any other "abominations" (such as vampires, werewolves, and witches)." this would not only have given us a main quest, but Sorine and Gunmars quests (or their replacement's), and also quests involving daedra, witches, werewolves and all sorts of nasty creatures.
The quest could have started with the Dovakhiin defending the hall, and making sure it doesn't get destroyed.
The second Faction they could have used in the Dawnguard's place is the http://www.uesp.net/wiki/Oblivion:The_Order_of_the_Virtuous_Blood_%28faction%29 If you've played Oblivion you'll hopefully remember them. Yes, they were abysmal at actually combating vampires, if you've done the quest you'll know why, if not click the link. But a lot can change in 200 years, and surely having the champion of Cyrodill in your order can only improve them. But alas, we have no mention of these guys at all, all that history...wasted.
Discuss.