I'm really a lightwave user who just got vue as part of my upgrade, but I did see a few things on this while messing around....  I think what might be simple enough is to use a temporary box on your path.  Vue has a density contol that has reference to "Foreign object"  in other words just dropping box to sit on the path, then press populate... Vue will not place any trees near the box.

Alternately I noticed that a grey scale image can be used to control population. I suppose if you render an overhead shot of the unpopulated ground surface, and then convert the image to black and white (invert if necessary) tress should not grow on blank areas