Forgot, sorry.
Your idea of plugging and unplugging the vacuum advance hose is going to tell you what the vacuum advance is not the mechanical advance.
Say you have the car at idle, the vacuum advance hose connected to manifold vacuum and your timing light shows you have 32 degrees of advance. Then you disconnect the vacuum hose from the vacuum advance can and the timing light shows you are now down to 14 degrees of advance. 32 degrees with the vacuum connected minus the 14 degrees with vacuum not connected is 18 degrees of vacuum advance.

Now if you include the theoretical mechanical advance of my last post, we now have:
14 degrees initial;
24 degrees mechanical;
18 degrees vacuum.
Now at cruise speed, if you are cruising at an RPM above the point where the mechanical advance is all in you will have an advance of 56 degrees. Good for mileage and a cool running engine. If you decide you want to 'floor it' to pass someone the carburetor's throttle plate opens all the way, causing vacuum to drop, and you are left with a total advance of 38 degrees. Total is initial plus mechanical.

I'm not saying these are ideal settings, just theoretical numbers to try to explain a little about mechanical and vacuum advance. Some cars run well with 12 degrees of mechanical some run well with 20. Depends on the set up of each engine.