VPD = Vapor Pressure Differential
This is an involved measurement that solves for the ideal conditions for plants to open their stomata and breathe without fear of wilt. Properly satisfying this condition is a prerequisite for CO² argumentation to have much effect.
In the chart below, the green boxes represent the temperature and humidity combinations that are best under VPD theory. Choose either temperature at left out humidity across the top, read to the green box, then up/across to give the corresponding value.
For example, running a bloomroom at 80F and 70% RH sounds insane- but according to the chart, it's right on the money.
I hear the 'omg, everything will rot!' cry even now, and the bottom line is that air movement is your friend no matter what your humidity levels are. Make your leaves dance and you're golden.
Drop your RH levels when you flush, this will encourage the plant to pull a lot of water through itself and finish transporting any excess minerals it may have, helps bring out fall colors and makes for a tasty smoke.