There's a lot of theories about the Vex but one thing that throws most of them off is the fact that the Vex are essentially computers and they run simulations and somethings they can't predict. In one of the cards it's mentioned a group of scientists studies the Vex and realizes they are running an infinite amount of simulations, they have worlds where you exist only maybe a major decision you made was different, or what you had for breakfast yesterday was different. They can predict every single outcome possible, and then take the one to the easiest victory. One reason why they can't understand Guardians is that Guardians are supposed to be already dead, and they can't understand that they are actually alive again. The problem with a theory of visions in the VoG is that the Vex could show Ghosts or Guardians simulations of what they predict will happen if they win, the only problem is Guardians keep screwing up the simulation because we are supposed to be dead.[spoiler]Sorry wall of text lol.[/spoiler]