If the sphere of a VDG was made of activated carbon I'm pretty sure it would have the same capacitance as a polished sphere made of metal.
You could just write a program that simulates the behavior of electrons in metal. Just a very simple simulation that works basically like a game where every object is moved a little bit in each iteration of the game loop. Such a simulation would only need Coulomb's law to calculate the forces between the electrons and it would allow the electrons to move within arbitrarily shaped boundaries which represent the surface of the metal. Just put e.g. 1000 electrons into the simulation and let it run.