I have to throw my two cents in on this one because I think you guys are making this to hard. Remember, these are old engines - they operate on simple principles.
To get right to the heart of manifold vacuum, you need to understand two things: 1. Where does the vacuum come from? 2. Why does vacuum change?
1. First it is important to point out that some might be confusing manifold vacuum with venturi vacuum - this is how your carb mixes air and fuel in the proper ratio. Manifold vacuum is created when a piston moves downward during the intake stroke, thereby expanding cylinder volume above the piston and creating an area of low pressure. As a result, air (and fuel) is pulled into the piston through the mainfold and carb. Since the pistons move up and down at different times, manifold vacuum is essentially constant.
2. Engine speed is limited by the amount of air (mixed with fuel) that is allowed into the pistons - this is controlled with a simple butterfly valve below the carb. When the throttle is opened, more air (and fuel) is allowed into the pistons and the engine speeds up. In theory, vacuum would entirely disappear and the engine would operate at atmospheric pressure as the restriction to airflow (the butterfly) is removed. Note that the carb would still work because, remember, it operates on the venturi principle. In practice, however, we all know that the engine maintains a vacuum. However, it is not always clear what will happen to vacuum as the vehicle accelerates - this is a function of several variables, but is largely determined by the load on the engine and the parasitic drag that the air (and fuel) encounter as it flows through the carb and manifold. Two examples: 1. Remember the last time you floored the old bolt up a big hill with a load in the back? What happened to vacuum? It was probably almost nothing. Why? The pistons were moving up and down more slowly which creates less vacuum to start with. However engine speed was limited by load. Since the throttle was wide open, and there was no restriction to the amount of air that could flow in, vacuum pressure should approach atmospheric pressure. 2. Now consider the last time you were late for work and you flogged the old bolt down the highway. Vacuum was lower than what you see at idle. Why? Theoretically, it should again approach atmosheric pressure since the restriction to the airflow is now removed (i.e. throttle wide open). In practice, however, parasitic drag (turbulence) in the manifold becomes the limiting factor to airflow speed and a vacuum is maintained.
Odds and ends: If my rambling made any sense, it should be pretty clear how a turbocharger or supercharger works. Both systems work to increase airflow into the engine. Imagine you have a turbo that produces 7 lbs of boost. What that means is that your engine (at full throttle) will be operating with 7 lbs less vacuum than it would without the turbo. In many cases, this means the engine will be operating above atmospheric pressure.
