Vapour-pressure deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. ... On the other hand, as the VPD increases, the plant needs to draw more water from its roots. In the case of cuttings, the plant may dry out and die.
But VPD can also be used as a driving force in transpiration and help us reach peak performance of our environment by harnessing peak transpiration flow through our plants
VPD does vary slightly on latitude or negative pressure so these ranges are provided as a guide
and for you old school folk
and a combo chart for anyone else