Depending on the distance of copper run, you might find that the added computation complexity of combining both conductor material types is insignificant. As I remember, Copper usually carries 40% more current per size than aluminum. So, this would make the aluminum run "the point of largest impedance". Although the copper run will have a VD value, by percentage it will be very small compared to the aluminum. I would extend the VD computation an additional "X" feet with aluminum factors to build a nice conservative circuit.
However, with that said, if the copper run distance is larger than the aluminum, then disregard the above. If you post the distances I'll compute the total VD using IEEE exact formula for you as this formula is a bit complex.
Also, don't forget to allow correction factors for ambient temperature in your VD computation. The NEC tables are set at 30ºC(86ºF) which is not practical anywhere in the USA. I personnally use 40ºC(104 - 105ºF) as a standard ambient with additional temperature rise for attic and roof mount circuits. The Chapter 8 and 9 tables are "borrowed" from IEEE Std 141 and are derived at 75ºC.
Not to sound like a commerical (this is really true), but using software that takes these and other factors into consideration is a lot easier and more accurate for VD computations.
