I feel like I should know this but I'm stumped! Take a look at this image. It's an example of a VPL emitting green light as it has struck the green surface of the Cornell box (the black surface in this image). How do you ensure that the surface that the photon has struck (and is reemitting the colour of said surface) doesn't get lit by the VPL?

homepages.paradise.net.nz/nickamy/realtimerad/oneindirect_shadows.jpg

I think you have to treat its VPL as a hemispherical light, you just include : dot(light_normal , vec) in your shading equation where vec is the normalized vector from light's position to the position of the point being shaded.