Thought I'd chip in w/ my own observations, based on the comments I've seen in this thread...
VCM can be achieved fairly easily in the car. The specs say that it can be engaged from 20-82mph, although I've been able to get it to kick in from a dead stop at about 18mph. It will hold this to just above 15mph, but the engine will lug noticably - thus, I try to keep about 20 or so when I'm in the office park where I work at.
There are two main ways to engage it...
The first is when you back off the accellerator ever so slightly (you're signalling that you want to cruise). The ECO light immediately goes on, and if you accellerate slowly, it will remain on. If you goof up and take it out of VCM, it'll go right back in as soon as you ease up on the go pedal.
The second is accellerating slowly from a dead stop, and you haven't backed off the accellerator, but the ECO light comes on just after 42mph (that's when either 5th gear occurs, or the torque converter locks up - I'll guess the former, based on my '01 AV6 and it's shift points, and the fact that I detect what I think is the TC lockup at about 48mph). I can still be accellerating, w/ the ECO light on.
I almost never get to call IMA for boost - usually when I see it come on is:
a. When I get to almost the crest of a hill (wish it came on a little earlier, so I wouldn't lose so much velocity going up!), I see about 1/4 boost (as opposed to 'feel') and it immediately goes off when I crest. It's generally not on for more than a second.
b. After I've started up from a stop with a lot of regen (either through engine speed or braking, and usually both). I'll see the boost come on quickly about 3/4 of the way as I get to the end of the accelleration cycle.
From what I know about the engineering of the planned SoC, I'm thinking that it's just 'bleeding off' the excess charge given to it in that state. (the system tries to maintain between 80% / 20% of max / min charge level), and I almost always have 4/6 bars in my BAT meter (I live in a suburban / rural area, and I take routes that let me continue with as few stops as possible).
If you drive in stop-n-go, you'll eventually stop seeing this bleed-off, as it's using the stored charge a lot more for auto-stop, etc.
This second scenario is what I think Lewis is experiencing in his 'volunteering' scenario that he describes in the earlier post. With my driving habits / terrain on my usual routes, I almost never get to use IMA boost with any degree of effect or regularity (save for the scenarios I describe above).
I think I've said this in an earlier post, but it's worth repeating, IMO... the greatest efficencies to this car's FE are not to be had with the IMA system, but with VCM, especially given how easily it engages, and with practice, stays engaged.
IMA is valuable, but I'd give the nod to VCM over IMA about 55/45 w/ regard to FE improvement.
My next exercise is to find out where the discrepancy lies between "ECO always means 3 cylinder operation" and "ECO means most efficient usage - but not necessarily VCM-enabled". I've seen both stated, and I'd like to know definitively. Sometimes I can feel the kickdown to VCM, other times, I can't. So I want to see some more authoritative info on this.