In short, VRF puts fertilizer where it can be utilized by the crop. Theoretically, VRF is environmentally sound, but if it doesn't increase yields, reduce costs, or both, it'll never fly. Weather - namely rainfall and temperature.
Yield is determined by the factors most limiting to productivity.
In many instances, it is not. Soil fertility and the amount of water available to the crop are usually the two main factors affecting crop yield potential. Topography is a primary factor contributing to soil variability, and influences both fertility and available water. Thus, soils on similar slope positions tend to have similar qualities. However, unpredictability of weather creates unpredictability of yield responses.