Hi Thomas,
One solution I used to make a material map (same issue - I needed to cover a wall with ivy but didn't want all the additional polys for vray.)
First, I took a leaf component from an ivy model I had (17,000 faces with just one component!) - then used component spray on a rectangle in an empty model. I added and deleted leaves until it looked how I wanted to to look in my full model. I set up a view straight on, and with the right sun angle, did a vray render. As a result, I have a png (with transparency) that I can use as a material map in vray (I also made a bumpmap layer with a b/w version of the image.) Like Csaba suggests, I place it at a distance from the backdrop so it casts shadows. Here is an example in a rendering.
[image: penthouse_img31crop.jpg]
-Andy