I don't know what the protocol is for vehicles and ground control, but one would have to think that if the aircraft was that close, someone on the truck would have been able to see it approaching. How on earth did they not see it?

This is a horrible and 100% preventable tragedy. My heart goes out to the families of all involved, and particularly the families of the pilots and of the controller, who seemed to be burdened with far more than he should have been.