This is easily done in Vue 5, 6 ad 7 although it's a bit more automatic in Vue 7 because you can link the sun direction/orientation to the material automatically.  In Vue 5 and 6 you have to key in the sun rotation values.  Here's how.  Create a mixed material with the two materials you want to use.  Set up the distribution of the materials as follows in the function editor.  Basically, you are just doing a dot product of the sun's rotation against the normal (world standard) of the object.  In Vue 7, you can link the sun's orientation into the function editor automatically -- and when you move the sun around, it will automatically update the material distribution.  In Vue 5 and Vue 6, you have to manually key in the sun rotation coordinates when the sun is moved.