Just got back from our 1400 mile round trip out to Ohio. Some observations on the ECO/VCM for what they are worth.
1. The ECO light does not always match the VCM condition. The ECO seems to indicate when the engine is running at it's most efficient state given the demand for power at that moment except when you are accelerating under torq. That said, obviously the ECO light will be on whenever the VCM is running the engine at less than 6 cylinders and there is no demand for torq/acceleration.
2. The long flat cruising zones on I90 across NY using cruise control allowed us to watch how the VCM seems to opperate. At a fixed speed of 70, the VCM will use 3 cylinders to maintain speed on flats and obviously downhill. The VCM will use 4 cylinders to maintin speed on minor elevation changes but only until the cruise control demands more and then the VCM will go to 6 cylinders but that was rare. The ECO light would stay on except when the VCM demanded 6 cylinders. Whenever I wanted more power to pass or to merge, the VCM gave me 6 cylinders and no ECO light.
3. If you can watch the trip computer during this process, you can see the changes in fuel consumption with the VCM change-ups but bear in mind, the VCM wouldn't be adding cylinders if there wasn't a demand so it is not "JUST" the VCM that is changing the real time fuel consumption number, it is demand driven as well.
Hope this makes sense and helps!!!