Silly Timing ???????

Outlaw you are correct. With very few exceptions the vacuum advance on old mopars is connected to a ported source.

If you have vacuum at idle then the throttle blades are open to much exposing the port. Whether the line to the distributor is connected or not should not make a difference. If it does I would be looking for what the problem is.

Also, the vacuum advance is used to add timing under low load cruising conditions to improve economy. When the throttle is opened abruptly and under WOT acceleration manifold vacuum is close to zero well below the point the advance pot will respond to. Once you settle back into a steady state throttle position and manifold vacuum increase the pot will add timing.

Timing verse rpm is controlled by the springs and weights in the mechanical advance. From a pure performance stand point the vacuum advance is not required and you will find that pure performance distributors do not have it.