As far as i understood, you have precice elevations of your terrain. So you must have precice coordinates of the places where the vegetation has to be. If you do want to do it visually everything stands or falls with the capabilities of your system (graphic card). If this can handle OpenGL in a good way, you should be able to position the vegetation - in combination of top and side view - easily. If a software in this price category can do something like this, then i think Vue can do. Especially because of this drop to next object lying under the selected one feature (str-D). But i can see a complete other problem. You seem to be very brave! You want to use a complete new and therefore unknown software first time directly in a bigger commercial job - as far as i understood. I personally would normally always try to practice at least half a year on any 3D software before doing the first (more complex) commercial job. Hope that we can see the results one time. Good luck in the meantime.

One day your ship comes in - but you're at the airport.