Hi,
There is close to zero chance the gasses from the clean, kiln dried pallet material I used, can make its way into the interior bubble. The vapor barrier does all of the heavy lifting, It's essentially, super heavy metal foil. That said one can never be 100% certain of zero off-gassing.
The cedar in my sauna is in fact clean, kiln dried and unfinished (smells great).
For what's it's worth I can almost guarantee, that if you peeled up the wood in the saunas that you mentioned, you too would find a vapor barrier. It's a building code requirement. Otherwise the structure the sauna is inside would be subject to escaping moisture and eventually rot.
Finally, when a sauna is correctly built, the vents...by design, will circulate outside air in and heated sauna air out.
By the way, here is the latest on my sauna:
Hope that helps.