It is correct in the case of the MTM RS6 Clubsport that it is solely an MTM VIN !
You should see the car as a complete prototype. One of its kind machine. Because the MTM RS6 Clubsport is nothing but the Gumpert Apollo 000.
The reason this car has no Audi VIN, it is because this RS6 never left the Audi Factory, actually the Quattro Gmbh Factory, as an RS6. All the parts (most of them not from Audi RS6) were purchased separately and put together along all the MTM Parts at MTM Headquarters in Wetstetten.
To my knowledge, this is the only car which has an MTM Vin. Others has an Audi VIN.
Frederic