Sounds to me more like your distributor curve was not setup properly and plugging the vacuum advance straight into manifold vacuum does a patch job at hiding the hole in your mechanical advance curve.

FWIW on my jeep I went and gave it as much initial as I could before the engine rpm would start slowing down from too much timing. The old 4.2 inline 6 is a worn out pos anyway. After doing that I tried hooking the vacuum advance up to manifold vacuum because the carb I'm using doesn't have ported advance. Plugging it in made the engine die. It just couldn't tolerate any more advance at idle!