I don't know if I'm alone or this is everyone here, but pretty sure savanthun controls the vex, as it was her will that made Crota let them in to our world, they are all controlled by "one mind," they are said to be, "beings so evil it despises other evil," similar to the way savanthun despises oryx for his power of the deep. Also on top of all this, have you ever noticed similarities in the vex and hive? They are the only alien species' to worship their gods to life (Black Garden, Blades of Crota).