Scenes fail to render if they contain xrefs with VrayLightMtl with Direct Illumination enabled. Tried it with a simple scene containing a teapot and no go. Render hangs at Transforming Veticies. Rendering the teapot scene works. Just not the teapot scene xrefed into another scene.
Better to use a VRayLight in that case then. The light material internally adds a light before the rendering starts, and perhaps that messes up with the X-Refs.
Best regards,
Vlado