Are you reading a ported or non-ported vacuum? It sounds like you are taking the reading in the ported port for the distributor. Move the gauge to the intake like Roy mentioned to get engine vacuum. Ported vacuum only has high vacuum at higher RPM as it works on a venturi principle like a pitot tube. As the incoming air rushes past the venturi orifice, it creates a mild vacuum.
the engine vacuum (non-ported) is affected by the engine compression ratio, quality of rings, quality of valve seals, rpm, displacement, cam shaft profile, ignition timing and intake tract sealing. It will not be affected by the idle fuel/air mixture.
