Yes, the vacuum advance is entirely separate from the vacuum advance and yes, to check your mechanical advance disconnect the vacuum advance. Pull the vacuum hose off the vacuum advance can and plug it.
Then you will need a timing light with a dial back feature or have the harmonic balancer marked with degrees of rotation before top dead center. Easier to use a dial back light.
Start and warm up engine. Set your idle rpm somewhere around 750-850 RPM where it runs smoothly. You want the idle low enough so the distributor's mechanical advance has not started to move.
Point the light at the timing mark on the front cover and note what the initial timing is. Let's say it's 14 degrees.
Rev the engine until the timing stops advancing, Turn the dial back feature of the light until the gun shows the timing mark at the same 14 degrees you had when at idle. Whatever the dial on the light is set to when the mark is at 14 is your amount of advance.
If you get a harmonic balance tape to show the degrees on your balancer or mark it yourself. You start the same way as above, pointing the light at the timing mark and noticing the degrees of advance at idle. Let's say 14 degrees. Then you rev the engine until it stops advancing and note the amount of degrees the timing mark lines up on. Let's say it stopped advancing at 38 degrees, 38 degrees less the initial 14 idle degrees will be 24 degrees of mechanical advance.