GM uses ported vacuum for the advance system. So you need to plug the vacuum line from the distributor to the port above the throttle blades (no vacuum at idle). When the throttle is openned (off idle) the distributor will advance. Pretty simple.
Connecting the vacuum advance to the manifold makes no sense. If that is done then the timing will retard when the throttle is openned which is the opposite of what is needed. Plus the mechanical advance will be advancing at the same time the vacuum is retarding. What is the point of that?
Fords use the manifold pressure for their vacuum advance system. It this case, the high manifold pressure, holds the vacuum advance in retard. When the throttle blades open and manifold pressure drops then the distributor will advance. Different method but the same effect.