I'm just curious: Why no vanishing point perspective on icons?
Because icons are painted to represent not what is earthly, temporal and eathbound, but what is heavenly, perfected and spiritual. The "inverse perspective", the absence of shadows, and the flatness of the compositions in icons are all ways of expressing what is not of this world.
It is a common misconception that the iconographers of the early Christian period “couldn’t draw or paint”, that this was a primitive or naïve art form. In this regard, it must be remembered that the Byzantines were the descendants of the Greeks and Romans who gave the world the physical perfection of Classical sculpture and murals (such as the sculptures of Praxiteles and Pheidias, or the frescoes of Pompeii), and where the development of geometry allowed the refinement of linear perspective in depicting three dimensions on a flat surface.
By contrast, icons attempt to express the opposite of earthly "realism".