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The first thing is that higher vacuum doesn't always mean you are burning less fuel. It's a way to measure engine load. For example when you said you can drive at 18-19 inHg in 3rd gear or 15-15 inHg in 4th gear, the lower RPM gear (4th) will be getting better fuel economy at the same speed, even though the engine load is higher. 5th gear would be even better if the engine has the power to lug it along at those low speeds. You could see this if you had a MPGuino or SG with instant MPG readout.
You generally never want to accelerate at full throttle. Most cars will go into open loop mode, enriching the fuel mixture to protect the engine. I would say you should accelerate from 2000 RPM to 2700 RPM at about 5-10 inHg. Roughly 80-90% engine load.
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