You can't texture a ground plane with rocks in the room and grass outside, too much trouble. Why don't you just use the ground plane which is called infinte plane in Vue, and give it a grass material, and use a primitive plane which size you would adjust to match your room, drop it on the infinite plane, and give it a rock material. I must say that if you use planes, your materials will not look very good, very flat look, no bump. You could use terrains, with a round clip filter, at least for the inside floor, which is in the foreground.