Vacuum can be used as a relative measure on how much load is put on the engine - how hard you're asking the engine to work (or how much throttle you're giving it). As you put more load on the engine, vacuum drops. When you're coasting in neutral, there is no load on the engine (other than internal friction and weight) and so vacuum is at its highest point. When you're at WOT, you're putting a big load on the engine and vacuum will drop to zero.
So, you'll consume less gasoline driving at 4k rpm with 14'' vacuum than you would be at 2k rpm with a 0'' vacuum reading. Think about the gas pedal position in both situations... in the first, you're just using enough gas to keep the car cruising at 4k. In the second, you're accellerating from a low rpm and your foot is down.