What did the Vasto Lorde do before Aizen showed up?
Barragan was probably controlling them, as he was the king of hueco mundo, and he did technically create vasto lordes. To explain that, he was the first person when he was the king of hueco mundo, along with a few other adjuchas, to become a vasto lorde. Either that wasn't too far from Aizen becoming ruler, or they just kinda chilled in Hueco and got more powerful, killing random hollows or whatever.