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I feel redpoint5 and elhigh have very thoroughly explained how vacuum works. For practical purposes though, all else being equal, more vacuum means you're demanding less power from the engine, and thereby using less fuel. Although your engine is less efficient at lower load (higher vacuum), you're still burning less fuel.
Ideally, you could shift to a taller gear, thereby increasing load and decreasing vacuum, but transmissions have limits in their maximum gearing. In the same gear, higher vacuum means you're burning less fuel, even if efficiency is lower.
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