hein is wrong, Vue can read the tiff format. If this was a texture problem, Vue would ask you to point to the right texture.
The problem with the obj format is that it needs an .mtl file to export with textures. This mtl file contains all the paths to the textures. Now XFrog 3.5 writes this mtl file, and obj export fro Xfrog 3.5 import in Vue fully textured.
My guess is that XFrog tune doesn't write this mtl file.
If you're a member of the XFrog yahoo list, ask there too.
If you have XFrog 3.5, why don't you just reduce the poly count directly from the bottom of the interface?