I like to think more of displacement per mile than RPM per mile. For example you could have a 512 stroker turning 2000 revolutions per mile (a gear/tire combo that gave you 2000 RPM @ 60 MPH for example) and displace the same air and have the same basic internal friction as a 383 turning 2674 revolution per mile (or RPM @ 60MPH). The two should get very similar MPG and put similar TQ to the tires with identicle top ends, cams...

Also the whole thing about keeping the vacuume as high as possible is sort of bologna, yes you want to keep it as high as possible within a given combo but keeping that number down through gearing is not really by it self a bad thing. You will always get higher vacuume in 1st gear than 2nd 3rd 4th... but you will absolutly not get your best MPG driving everywere in 1st gear. The reason you were getting better MPG watching that gauge is because you were not doing as much work accelerating and then slowing down if you keep the gauge steady with a particular load if you were trying to keep the vacuume up. Also it is a good indicator of your engines tune state. I always got my best MPG with my 318 magnum 5 speed dakota by running a much higher gear selection than most would ever consider running and then accelerating with WOT at a low RPM. I could get near 20 MPG in town all the time driving this way, you will not get anywhere in a hurry but it saves a butt load of gas. If I ran it up to 2500-3000 to shift and accelerated at part throttle is would get maybe 12MPG in town.