Hi Bill,
I may have misunderstood you but the vacuum advance distributor DOES NOT use manifold vacuum. It is connected to a port on one of the carbs that has variable vacuum under various throttle positions, including zero vacuum at idle. That is why a vacuum advance distributor will have no effect on your rough idle problem. A vacuum advance distributor advances the timing at partial throttle which has the sole effect of increasing your fuel economy at cruise. It has no other effect on the engine except to give you better gas mileage.