According to my owners manual, the EC switch controls the source of cool air. In other words, when the EC switch is depressed, the automatic climate control uses outside air when cool air is needed instead of turning on the AC compressor. This results in greater economy since the AC compressor never comes on. All vents continue to operate as usual.
All the other control buttons , those with a solid white downward pointing arrow, utilize the AC for cool air when the climate system decides to cool the car instead of heating. Of course, the far left button is for the defroster and the AC is always used for quick defogging.
The climate control system was pretty confusing at first. Pressing a given button did not necessarily do anything depending on what the climate control "brain" decided needed to be done. There seems to always be a time lag between the car being too hot or cool and the climate system trying to do something about it.
Your idea of a vacuum operated switch to turn off the AC under acceleration is a good one.
Years ago I saw an accessory switch for sale which did that. Since I often drive underpowered Volvos, I've thought of trying to find or make such a switch. What is needed is a vacuum operated switch mounted to a T in a vacuum line which comes off of the intake manifold. This switch would be wired in series with the AC compressor clutch or maybe use a relay. The switch would need to be adjustable to find the right vacuum level for the AC to cut out. As you point out, when you press down the accelerator, the vacuum decreases and the AC would be disconnected.
This set up would not improve economy much but would direct more engine power to the wheels.
If you proceed with your idea, please let us know how it works.