This is a quick visual run-down of how to use the Matte comping tag properties for merging your model into a photograph within the vray renderer.

Image A is the pre-rendered background image. This image is put into the Environmental Background slot as Frontal.

Image B is the scene. A simple set of balls with a plane object ground.

Image C is the basic rendered scene. No reflections, just basic shadow and GI. The ground has no material yet, just a VR Comp tag with all Matte Properties activated.

Image D has all reflections activated, but suffers the ground object reflecting the background image. You cant switch off reflections from any of the scene objects as that effects the rest of the scene. So, a barrier is needed to mask out the background environment.

Image E shows where a vertical plane has been inserted into the scene. It's positioned just behind the spheres. It only has a VR Comp tag assigned to it, with Cast Shadow and Generate GI switched off, and all Matte Properties switched on.

When rendered, the lighting, GI and more importantly the reflections, are now all correct.
The background is masked off, but still provides scene reflections via the Environment Reflection channel.

Attachments: 

matte.zip