Your car only has mechanical advance.
The vac is Retard only
The correct timing procedure is to time the car without vac hooked up at 30 degrees btdc at 4500 rpm.

The vac retard works when the throttle plate is completely closed at idle. This is a retard/deceleration system.

The difference between vac and no vac at no load/idle calls for 8-14 degrees. This is adjustable on some distributors [ don't know which # you have], by shorten/lenghten the vac control rod
to the dist base plate. I find they run best with a short [ 8 degrees] adjustment.
If the difference at idle is beyond these specs, i would look into the base plate support bearing, mounts and also check the mechanical springs under the advance plate. These are known to wear and break off the tabs and a broken one[ there are 2] will over- advance the plate as soon as you remove the vac line at idle because
there is no tension to hold the centrifical wieghts. { That is what the springs do]

Recommend taking a good look at the dist. , from mounts and adjustments to wear and broken /worn parts.