I had a quick look at your scene and it seems that the solution is to turn on caustics in the "Render" settings. I have done this in your scene. You then need to render with "Standard" mode rather than "Fast Preview".
The reason for this is that the custom material you made is a perfect mirror. This means that the reflected light is specular -- it is a caustic. Specular reflections are not handled in the Fast Preview rendering mode of V-Ray. You need to both enable caustics and you have to render in "Standard" mode to have them show up. It looks great with caustics.
The reason why the "Material: Reflector" appears to work si that it isn't a perfect mirror, it is actually rough, not a perfect glossy material, and thus the reflected light you are seeing is difuse, thus it is rendered in the Fast Preview mode. If you make your customer material rougher, using the V-Ray Standard material you can replicate the behavior of the Material: Reflector.