I’m thinking VPD should correlate better because it predicts whether and how much condensation will form on the plant. Increasing condensation should cause increased fungi and mold growth, up to a point, anyway.
On the other hand, warm condensation should be more conducive to fungi and mold growth than cold, so I’m not sure which is the better predictor.
Does anyone know?
If VPD is in line then rH should be, but for late flower, lower rH than VPD. Since considering botrytis, I’d be concerned with rH. Just a guess for my opinion. Will be interested in what others have actually researched.
Air flow and exchange has a big impact. Fresh air circulating from your lung room can help immensely in decreasing mold chances. If the air is stail and there is no circulation, throwing in a high RH is a sure recipe for mold and other fungus.
Wasn’t an option above 
All day Facts!
IMO… The first thing to consider when talking about VPD is humidity. Humidity should be the target, and then you should match temp that supports that humidity. This can be complicated. You need more control over environment this route.
If you are running higher temps, you can get away with higher humidity, but the risk is still there and very present. Mid 80’s temps going passed 60% is still a risk.
I’ve never gotten into VPD. I maintain rH for stage. Never have run into mold inside. Figured when I do then I may upgrade a controller for my gen2 fans. The gen1 ACinfinity’s don’t work on others.
I prioritize humidity also, but when push comes to shove I’ll try to get in a safer range manipulating temps.
It’s always easier to increase the temps and lower the Rh. I usually bump up the lights. I also have heated floors in the tents maintaining 75° root zone. And when my RH is off then it’s time to water.
Edit. Combined with all the factors above. Filtered intakes and lots of airflow plus my ac is always running, so that’s my dehumidifier.
Maybe a different way of thinking about the question will help.
If the VPD surrounding the plant is > 1, there should be no condensation on the plant. Can fungi and mold establish colonies on the plant without condensation?
Spores are in the air unless you’re running a heppa filter they’re always going to end up on the plant so circulation and healthy immune response play a roll
I see @Hoppiefrog on rn.
Absolutely , just takes humidity above 45% in a microclimate or even a gnat at 40%. Assume theres always aspergilisis and botrytis everywhere
Even then hyper heppa is the only guarantee on bacterium , almost all spores but coleria and ecoli are fine w 0.3 micron filters but stuff like ecoli still is not 100% filtered at 0.003 microns (hyper heppa) tho its darn close to getting everything (99.5% on the ecoli)
It doesn’t matter that pathogens land on the plant if they can’t grow there. I’ve suggested that a VPD > 1 ensures that, and you seem to be saying – nope – it’s the RH, and it must be < 45% (or maybe it’s 40% – the gnat lost me).
Is that right? What do you base your criterion on?
Spores can pull moisture from the air if it’s humid, moisture released through transpiration, microclimates throughout canopy running higher humidity. All a risk even without condensation. Running lower humidity all around and mitigate the risk better than following VPD.
Even if the VPD is > 1?
PS - Maybe it’s time for you to break out the Chat GPT! 
I think so. VPD helps plant transpiration and reduces the likelihood of condensation, it doesn’t eliminate the risk if the humidity is consistently on the higher end. I’ve ran higher humidity and higher temps throughout flower mid 80’s. VPD was ok, not perfect. The single largest bud got mold. Higher humidity than I usually run but it was seemingly alright on the chart.
Edit
Keeps crashing, idk if they are updating or what but it’s acting different. Completely different vibes to the answers, mathematics have even been wrong lately.
Ill send a better response after i get up was up for a few mins earlier but got to go back to bed , but at first glance i don’t think you understand what a micro climate is .understand vpd will be way outta wack in said micro climate say inside a bud persay or under canopy or a area with poor ventilation. You would need a sensor inside your colas to see the big picture fully
I was told to use vpd until flowering. Once in flower ignore vpd and keep rh between 40-55%., temps 75F - 80F, or else ur risk for mold mildew increases, unless temps are above 85F which isn’t good for flowering.
I think you’re saying the VPD outside a bud can differ from the VPD inside. That’s plausible, but how large can that difference be? If the outer VPD is 1.2, for example, could the VPD inside be 0.8?
If you’ll look at the VPD chart I posted, you’ll see that your numbers stray into the chart’s red zone, although not by much. Do you recall the source that said to ignore VPD during flower and rely on the numbers you gave instead?
