Hi
I don't know these files but according to their names it seems that you should reorder the file names in your oconv command so that all materials definitions (apparently contained in the VDT.mat file) appear before the geometry files.
This means that you should simply change:
oconv VDToff.rad VDT.mat VDTscreen_off.rad > test.oct
into:
oconv VDT.mat VDToff.rad VDTscreen_off.rad > test.oct
Hope this will help you
Raphaël