I'm a hydro grower so my knowledge of soil is very limited but…isn't the drooping leaves the classic sign of overwatering?
A VPD of 1.4 means that you don't have enough humidity whereas lower VPD value indicate that RH is increasing.
VPD is the difference - the "deficit" - between the vapor pressure (the amount of water) inside the leaf compared to the humidity and temperature outside the leaf.
Loosely speaking, seedlings like it hot and wet - 80° and high humidity are what they need. As a cannabis plant goes into veg and flower, you want to keep the temps up and RH down because that helps the plants "respire" which is when that take up water (and nutes) and then breathe it out. That's how plants moderate their temperature and it also drives the uptake of nutes. If your VPD is too low, plants don't take up water which means that they don't get nutes.
Good example - the climate in the Southeast US is hot and humid. We humans feel that. It's "sticky". Move 2k miles west. It's dry and hot. It doesn't feel hot but, no doubt about it, that low humidity means that everything has to take in a lot of water to cool off. Plants are impacted in the same way.