Vue's material handling is very basic and object based, so unless predefined mat zones are present, it treats every object as a single material, hence the results you are seeing.

You would really need to map the cube in an external program or at a minimum, assign material zones to each side, though this too would need to happen in an external app. Alternatively, you could rebuild the cube using six thin cube panels right in Vue and then apply the materials of your choosing to each individual panel - I use this trick all the time as it's faster than going into another app for simple objects.