The vest would be good at blocking direct conductive heat loss but if the air pockets were big enough heat can still be lost via convection. Even if the air is 'still' the act of the air heating, rising, cooling, falling, expanding, and contracting will move it enough inside the pockets to dissipate some heat. Looks like the vest has a lot of air chambers and that will help a great deal at preventing air movement.

Better than a straight sheet of space blanket though as far as insulation goes.