Spoiler-riffic! I’m not going to tag it because no one else has anyway. But, there’s a warning for you.
The way I saw it, they were just devolved into a more primitive state that made them stupid, but still able to adapt. Like crazy monkeys or something. They had enough of a social structure to recognize relationships (The leader and the girl) and to form communities around a leader (Though it was obviously based around him being the biggest and baddest, and that’s why they followed him. Plus, I assume they only reason they knew to set the trap was that they had been watching him for a very long time and had enough of their kind set by the traps to understand that the mannequin would mean something to him and to learn how to set the trap for themselves.
Also, I don’t think they were meant to be vampires, just have vampiric qualities. Hence the reason why they are never called so.
I enjoyed the movie, by the way. 
Though the very, very end with the woman’s monologue didnt do much for me overall.