Generally, anything that helps your vehicle breath better will end up using MORE fuel unless you granny-drive it still. More air = more fuel.
Vacuum will tell you how hard the engine is really pulling, and gives you a good idea of how much "power" you're attempting to use.
Air/fuel ratio doesn't change much and you can't easily change it much in a MAF based vehicle with oxygen sensor feedback. If you try to lean or enrich it, the computer just compensates and if you take it out of the range it CAN compensate, your check engine light comes on.
Now, if you DO fool it, you can still screw up cats and what not. Probably not worth doing -- I'd forget about the A/F ratio gauge unless you are going to do radical computer tuning, or have a carbeurated vehicle!
The best things you can do are the traditional ones: driving style, tire inflation, etc. But a chip with two programs would be nice. An "economy" program and a "performance" program. That's a bit of an expense though. Figure on $300 to $400 for either that or a "tuner" that lets you "flash" your PCM.