'rgbGen vertex' should result in the entire thing basically being fullbright (iqm supports vertex colours, but they're a pain to export so I doubt you're using that, thus fullbright).
if the engine is already ignoring stuff like that then there's not really much you can do from the perspective of the shader.
you can try throwing more lights onto the map, to try to ensure that the base light levels are bright enough to light everything. the light grid has min+max+dir values, so you need to make sure both sides are light enough. I don't know enough about q3map2 to do this, try minlight? :s
remember the light grid is 3d, so you'll probably need lights both near AND far from the ground.
also, make sure your entity's origin isn't weird and inside the ground.