I can't do rolling harriers, but surely the angle has something to do with the forward speed you want, increase the angle enough and you stop moving. Also you must get to a point where the ailerons become useless like in a TR (as edible engine says). Surely any angle of attack above lets say 15 degrees is a harrier. You are still going to need rudder and elevator to control it. A roll is what happens when the plane is rolling around its datum line, it may be vertical even but its still rolling around its center line. But in a harrier its rolling around a line inclined 15 degrees or more from its flight path.