VPD seems to be tossed around a lot with few following the actual protocol. I don't think it should be applied with a small grow and the inexperienced in general as it can be counterproductive then. Most that adhere to this have proven cuts that have extreme mold resistance and a lot more experience with this all then say the typical home growers just looking to produce enough for personal use.
Back when I had nothing but a closest to grow in during my youth I didn't have the advantages of ventilation, let alone room for oscillating fans. After moving forward in life and building larger setups I began to note that some things did really grow better in that old humid closet and it was VPD always at the constant perfect value even though I lived in what seemed like a jungle. Same thing was observed with the specimens that I took home from the greenhouse I worked in back then regardless of how much light and feed I supplied.
Ironically, the only time I have ever had issues with high humidity and mold was with the modern mixes and towards the later of my years growing inside and out. Never had issues with the old land races, cultivars as well as even the earliest of commercialized stock obtained in the early 90's. With today's poly x poly and the incestuous trends I find that running with the VPD dialed in can be a hit and miss due to lack of resistance that isn't always "strain dependent" as it is then Pheno dependent. Even the Mexican hybrids of the 90's were very resistant and strong in comparison with the mutts called hybrids today.
Perhaps the axiom of bigger is better is the problem, it's an American thing for sure. Bigger buds equals bigger risks on an average indoor and outdoor. This is another thing I believe is a factor in this all.
Big Bud, it was a very popular type for a reason and the trend never died, even if the
big bud hype died quickly. Everyone seems to want the biggest dense of flowers, even though on average high quality cannabis is moderate yielding at best. I have seen plenty of the modern types mold in 50% RH due to density and trapped moisture in the flowers regardless of air activity, yet again I have never seen it with the older stock.
So much of the paradigm surrounding growing cannabis can't always be taken for face value, subjectively each grower should be doing what works for them, their plants and the goals sought without adherence to the mainstream..