When you mash down on the throttle, your manifold vacuum will always drop to zero, then come back up when you steady out the throttle movement. Is your distributor advance hooked to Manifold or Ported vacuum? Pull the distributor cap off and #1 spark plug out, install a compression tester hose only into #1 Cylinder. Have someone else crank it over while you watch to see where #1 pop's off at. You could be 1 tooth, or 180 degrees off on your distributor timing causing that low vacuum at idle.
Looking for a penny for my 2 cents worth.