Maybe tie it with experience? Early in the war soviet tank crews had only basic training and they could not do even basic repair of their own tanks, so when something broke they just left it where it was. After they lost most of their starting tanks during early stages of Barbarossa, new tank crews were trained to at least do some basic repairs themselves. Also, more experienced crews tried to destroy vehicles so the enemy could not use them. So to keep it simple, tie it with relative experience - experienced unit fighting green one should recover much more vehicles after a successfull battle than experienced against experienced, and green unit against experienced should get the least. Also, conquering a nation should give the conqueror a % of the equipment pool.