Well, I can think of 2 ways...

After creating the scene as normal (in full colour):

1. (The easy way).  Render and save resulting pic. Import pic into (almost any) photo enhancement (2D painting) app. Convert to grey-scale...

2. (The not so easy way). Change all the textures on everything in the Vue scene to a grey-scale equivalent. That includes the atmosphere, the sunlight, all and any textures applied to any terrains/rocks/trees/water etc.. Then render...

There may be another way. But I don't know it...

Cheers,
Diolma