I have zero mold issues in a high humidity environment with no air exchange whatsoever, completely sealed environment. So I think it does come down to adequate air circulation. Stagnant air is the cause of problems!!I'm a mushroom farmer by trade and I think I can help contribute something useful to "why does vpd work?"
When you're building a mushroom fruiting room, you build it for positive pressure to keep it from sucking in dust and other contaminate particles like mold spores and bacteria. The constant positive pressure means constant fresh air introduction. The primary source of contamination in microbiology is STAGNANT (humid) AIR. That's why some folks are probably having issues, improper air circulation inside their rooms leading to stagnant gasses inside the buds themselves. Every part of the plant needs gas exchange to prevent stagnation. I've seen the same thing in grow rooms on farms I've done consulting for. Those same principles apply to cannabis growing As long as you maintain a constant positive pressure, you can maintain a mushroom fruiting room above 90% humidity with very minimal contamination from mold, mildew, or bacteria.