Very Good Post I rate this a 10!
The only issues we run into with Vacuum units is on the Mopar stuff, the advance plate system is junk as the vacuum unit starts to pull the advance in the plate lifts off the pivot point and causes misalignment of the reluctor to pick-up and on some well used units cause the gap to be compromised either direction, some to the point of reluctor collision. This causes the timing to wander and the idle to follow. As these old distributors get more and more wear and tear we've even had some where the plate will hang up and cause some serious detonation. The only sure cure we've found is to use all mechanical timing and set them up with enough initial 18-24 to burn the fuel and use the mechanical timing to bring the advance in as RPM builds.
This really only becomes a concern with big cammed street motors that respond instantly to throttle position. If it's a mild application or stock we set the vacuum controller to manifold vacuum to get a bump in the high vacuum timing. We take special care in selecting a good core without to much wear on the mechanism when we build a Stage 1 Vacuum distributor.
The GM distributor has a great vacuum plate that holds everything nice and square so this is not an issue, but our poor Performance Mopars need help.
We're working on an advance plate upgrade that hopefully will cure the problem but for now the all mechanical system is where we're at.
Race cars are another topic, if it's a race car only we don't mess with any vacuum unit at all, just stab the throttle and keep it between the walls their always at full advance before the convertor hits anyhow. I also find it easier to get a good plug reading for tuning if the distributor is set with just RPM timing, high idle timing from a vacuum unit can mislead you on plug temps.