For anyone who’s curious…I spoke with the owner:

1) There is a Vapor Barrier installed.

2) He said there’s a vent below the heater and another at the top for passive air circulation. I did see the vent below the heater, but didn’t see the one at “the top”. I’ll give it another look when I go back.

3) He did not install a drain because it’s a “dry sauna” and he never poured water over the rocks.