There is indeed a problem with especially the techique XFrog uses for transparency. They use tif files for their textures with embedded transparency IN the tif file (which i doubt follows the tif standards, it is a derivate of the format). And this Vue does not read correctly. But you can correct it. Open the material editor for this leaf material(s) and switch the editor to the simple mode (not advanced editor mode). There you should have three previews only (more or less). To the left you see the map for the leaves, the middle is the related bump map and to the right you see the transparency map. There you select the little tick "use texture as transparency" and then it should work as you expected it. I am sorry if i have not used the exact right words for the menu expressions, i am at home right now without Vue installation and do it completely out of my head. Can't double check.

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