Eagle... you will find that lighting on the alpha plane may look a bit off from the rest of the scene... you can either adjust your original image, or you can disregard the lights for that alpha plane and set it's illumination higher than 100%.  black robert from Poland has some great info on his website... http://www.graphique3d.republika.pl/

he has a nice tutorial about how he put his great VUE scenes together.  ... part of it is on his alpha plane sky settings.