A vacuum leak is only one of several causes of low vacuum. Ignition timing is one of the big ones. Valve adjustment is another, and as has been mentioned, worn piston rings can cause the problem. Do a cylinder leakdown test to try and spot any compression leak, valves or rings, and try advancing the ignition timing to see if it's just late timing. Also, use a Propane torch (unlit) to check all the manifold-to engine gasket areas for leaks. If you find one, the engine should smooth out or speed up as the gas enters the leak.
Jerry
