[RichardJ]

>> I've experimented with advancing it, in an attempt to rule out dampener slip<<
Put a chalk or paint line on the ring and also on the hub or any pulleys bolted to the hub. Start and stop the engine once or twice and your marks will be misaligned if the damper is bad.

If the problem is,
>>the idle vacuum has been low. ~10 in/mg. This has never really caused a problem<<
you needn't concern yourself with the vacuum advance. Disconnect the hose and plug the vacuum port at the carb. Vacuum advance is used for fuel economy. No need to have it working until you deal with the low manifold vacuum.

Does it have a points distributor? Does the dwell stay steady or at least within 1 or 2 degrees as you rev the engine.

My question for you, is 10" vacuum the peak vacuum as you adjust the idle mixture screws or what are you seeing as you adjust the idle mixture screws.