I believe you're talking about the large white vapor barrier boots, AKA Bunny boots, AKA micky mouse boots? They have been around for about 30 years and are still in use.

The valve is actually for high altitude use - like when the wearer goes up in a plane, to release the pressure inside the boot. They don't have anything to do with moisture release. The beauty of these vapor barrier boots is that they cannot get moist air inside of them inside the insulating part. They have a complete rubber inside (vapor barrier) , a thick felt insulation layer and a complete rubber outside. That's why they are a bench mark for extreme cold - not only are they warm, but although your feet will sweat, the sweat won't get into the insulation.Although yoru feet might be wet inside the boot, they won't get cold because the insulation is not affected.Your feet will indeed sweat though. I've found (been using them for 25 years) that it's always a good idea to bring extra socks and simply change your socks during the day. Although I've generally not bothered for a normal 6 - 8 hour day, even though it would have been more comfortable. If you're wearing them for a long time, several days in a row, you better make sure you let your feet air dry regularly or they will prune up and actually get very sore (needles and pins feeling).

I've used them for many long periods sitting or standing still for 10+ hours and my feet have never gotten cold. I've used them for more active stuff for days on end as cold as -40 and my feet have't gotten cold.

I do believe there are better boots nowadays, but 25 years ago these were by far the best, and still are a benchmark.