A "normal" vacuum advance for a carbureted engine shouldn't have manifold vacuum connected, rather it should be fed from a source of ported vacuum. Such that vacuum rises with engine speed.
The difference between ported and manifold vacuum is that ported has no vacuum at idle when the throttle blade is closed. As soon as the throttle blade is opened the ported vacuum port has manifold vacuum exposed. So the difference between connecting the vacuum advance to a ported vs manifold source is that ported will have no additional advance at idle, and a manifold source will have the additional advance at idle. The engine actually prefers more advance at idle except for hot starting, as noted by the higher RPM when given more advance at idle. The engine is more efficient with say 32 degrees of advance at idle vs 12 degrees of advance at idle. Since there is basically no vacuum when starting the vacuum advance is not added at startup.
When under load at higher throttle positions the vacuum decreases in the manifold and the additional vacuum advance goes away. So a sudden wide open throttle from idle means manifold vacuum goes to zero, and the additional advance from the vacuum advance goes away. As the RPM increases the mechanical advance adds additional advance. While cruising at higher RPM, say 3,000 RPM the mechanical advance is added, and the throttle is low and manifold vacuum is high, so the additional advance of the vacuum advance can is added, again, the engine likes more advance at low throttle low load higher RPM situations.
So for a typical situation at idle, the engine would be spark timed for say 10 BTDC, with no mechanical advance added at idle. Adding 20 degrees advance at idle with a manifold sourced vacuum advance means the engine is idling at 30 BTDC, which generally is just about in the optimum range for idle. Open the throttle quickly and the additional 20 goes away and the mechanical is added to the static 10 BTDC as the RPM increases. With the vacuum advance can connected to a PORTED source there is no additional advance added at idle, so the engine will be idling at say 10 BTDC. At cruise at 3,000 RPM the manifold vacuum is high, the ported source is exposed to the manifold vacuum and the additional advance from the vacuum advance is added.
The bottom line is that the ONLY difference between a manifold connected vacuum advance can and a ported source connected vacuum advance can is that the ported source does not add the additional advance at idle. MOST engines like to idle with roughly 30 BTDC so the ported source is not preferred. If your engine does not run better with 30 BTDC than 10 BTDC at idle it is probably a CARB problem that can't tolerate higher advance. Lean mixtures burn slower than richer mixtures, and an out of adjustment carb changes the advance requirements in order for the peak cylinder pressure to occur at the appropriate time AFTER TDC. Too little advance makes the peak pressure occur too late, having a negative effect on engine performance.