The vacuum advance line is between the carburetor and the vacuum advance unit. The vacuum is created in the intake manifold/carburetor. If you disconnect the line at the carburetor, you need to plug that "hole" in the carburetor. If you disconnect that line at the vacuum advance unit, you need to plug the open end of the vacuum advance line.
When your engine's idle is at normal idle RPM (around 500 RPM), that line does not have to be disconnected in order to set the timing. However, as long as you plug the vacuum line, it will not hurt taking this additional (unnecessary) step.
USA 1954 Truck Shop Manual instructions for setting timing. The same instructions are shown in the
48-51 Truck Shop manual.
1960 Truck Owner's Manual - ignition timing 
