Well, 15 does sound low. It could be ignition timing, but what sounds strange is the fact that the vacuum doesnt change with timing. Advancing timing will raise vacuum, retared will lower it. Is it possible you are hooked into a "ported" or "timed" vacuuum source that is affected by throttle position? Also, higher altitudes affect vacuum, but not that much.
Are you sure the gauge is hooked to intake manifold, which is the correct spot? . Ported vacuum is found on the carb, not intake manifold. Hard to give long distance help.
Can you give me some more clues? What happens to vacuum if you snap throttle? You say vacuum is rock steady except when idle changes. What does vacuum do at that time? Increase? Decrease? Choke engine by hand, what happens to vacuum? Lets try and figure this out.
