VPD is a measurement of the potential for plants to breathe in layman’s terms. It is not vital to understand it completely as you start growing many great growers ignore this and just sorta read the plants but make no mistake they are dealing with vpd like an engine mechanic deals with combustion. They never actually touch it or see it but everything they do serves it.
there are tons of threads out there on VPD.
Ok i was gonna sit down and write out a whole article but i have been lazy so instead instead I'm going to copy paste because im lazy. This is not my work so let start. You may find this very long but its worth the read. Some may want to skip ahead. I have added here and there to what i feel is...
www.thcfarmer.com
what controls we have on VPD are leaf temp and humidity. In early veg you want higher RH and as you grow and flower it needs to drop.
If you can’t measure leaf temps or can’t adjust temp or RH you just need to try to keep things in the range as best you can. Take a look at the thread and see if it answers your