The Incident Light Angle will not work when plugged into a node path which connects into the color channel.  The reason for this is the so called circular interpolation problem.  The Vue render engine needs to have at least a base diffuse color as a starting point when doing ray tracing.  All of the light input nodes in the Function Editor can only be plugged into the Specular output channel.  If you are trying to create a diffuse color pattern based on the angle of a light in the scene (e.g. a fresnel effect), you can capture the vector rotation coordinates of the light and then use a Dot Product function against the object normal and then plug tht into a color gradient.  I do this in SkinVue to create the SkinFresnel effect.  The only issue is that you will need to re-input the light coordinates if the light angle changes.