Ok, so I have this situation where the file doesn't want to render in Mental Ray as seen in Viewport under "Use All lights" setting.

The file you see in these images are in the ideal view in the viewport [1st image] and the view in mental ray [second image].

It's made up of a layered shader. The first of the two mixed shaders is a file and the second is a ramp shader. If I make the ramp shader above the file layer it turns black, but with the file above it: Together they produce a great appearance of a toon character with no shadows on it but can still receive light on it. An interesting feat that took me 3 years to figure out lol.

However, what stumped me was Mental Ray. It ended up showing what it looked like in basic viewport view and not the typical "Use All lights" view. The file in the layered shader isn't a solid shader, where it's circled in all lights and the ramp shader [without this file layer] alon can still receive this lighting in the render. So I'm at a lost on why it cannot use both or just the file in the layer shader by itself in the first place to simply receive light from a spot light [in this case, I colored it purple so it's easy to see with her skintone]

I'm hoping this will be a solution I can fix in the Mental Ray settings [or possibly any other way] since this lighting technique allows a toon image to have both a sharp and smooth shadowing effect like any other shader [without the need to have 'shade' on them of course]. However I'm stuck at seeing this only in my viewport. This solution, if discovered, can be perfect for flat-shaded 3D environments with lights littered in it like seen in animes or traditional animation.