Some anecdotal comparison:

Back in 03 I went through my hazmat level a certification. We had to put on an enclosed tyvek suit (not level A, but close) with an SCBA inside. This was in the middle of winter, and we were experiencing a frigid arctic blast at the time. Outside temps were around 15 degrees F, and winds out of the northeaast at about 15 mph. After we were suited up we had to walk from the classroom to the simulator, maybe 200 yards. Our escorts are wearing the full complement of winter gear; parka, watch cap, gloves, boots. Inside the tyvek suit, I am wearing a t shirt, jeans, and my work boots.

Now the tyvek suits are notorious for not venting well, and I am an air hog with an SCBA (I consume about twice as much as as the average joe over time, go figure), so my suit is inflating some (the suit vents better once a little pressure differential is established). Anyways, the whole time I am walking across the parking lot, I notice that the temp inside that suit is still warm enough that I am perspiring. Now once we get into the simulator and start crawling around I am sweating and getting uncomfortably hot. Once the simulation is over, I punch the bypass valve on the SCBA to get the cold compressed air blowing on my face continuously to help me cool down. After I have doffed the suit, I take a step outside, and my warm sweaty body jolts from the frigid cold blast.

So I am thinking that way more than 1/8 inch of air gap between my body and the suit did a lot to keep me insulated from the cold and trapped a lot of body heat in the process. From an experiential point of view, I'd say the vest must be similar.

Even though the air in the vest can circulate more or less freely within the individual chambers, I don't think convective loss is going to be a factor because the circulating air doesn't have a good thermocouple to the outer layer of the vest, based on how the air in that tyvek suit stayed so toasty warm. If the outer layer of the vest were metal or wet fabric then probably convective loss would be an issue because the circulating air in the chamber would be in contact with a much better thermocouple.

Come to think of it, once my suit inflated, I did get cold spots where my skin came into direct contact with the tyvek, such as at my elbows and wrists when I had to bend my arms. I suppose that, had the suit not inflated as it did, I might've ended up much cooler than I was. That seems counter-intuitive to the 1/8" principle.

Sounds like a job for mythbusters to me.