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The vacuum in the intake is caused by the throttle plate restricting the flow of air into the intake, while the engine is sucking from the other end of the intake. It can be used to do work (e.g., brake booster) but that is IMHO secondary. (Some smart engineer decided it was "free power".)
A vacuum gauge may be useful for getting fuel economy in that it can tell you (more or less) how much air is going into the engine. It gives you more precise feedback than you trying to figure out how far your foot is pressed down and how fast the engine is spinning.
A vacuum pump can be useful in a car that does not have vacuum-operated systems. In a number of high-powered engines, a vacuum pump is used to draw out some of the air from inside the crankcase of the engine. That means there is less air in there for the "back side" of the pistons (and for the crank and rods and such) to push around, which means that more of the engine's power can be used to move the car.
The same theory would say that it would require less fuel to move the car as well, since less of the work the fuel is doing is needed to push that "inside air" around. My feeling is that any gains from that will be too small to repeatably measure when you talk about the power levels that are used for steady-state cruising.
-soD
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