Hi FieldEffect. Thank you, I agree and hear what you are saying.
To help clarify what I was noting regarding VPD being an odd metric, the difference between using VPD on a live plant vs drying plant matter hides a piece of information inherent in the calculation.
Leaf temperature, as you’d noted, is one. Leaf temperature meaning plant matter in this context. The temperature differential is practically zero for this scenario (I believe). So we ignore that.
The other is humidity of the leaf. The humidity of the leaf (plant matter) for VPD is simplified such that the leaf is considered 100% saturated. 100% humid. It’s a simplification and you’ll see most online calculators gloss over that value.
Here are a couple of the preliminary calculations for VPD (full algorithm implementation detail can be found here):
vapor pressure atmosphere = Rh * Es
vapor pressure leaf = (1) * Es
The (1) in the above indicates 100% humid. As the plant dries, the plant matter has to become less humid otherwise it would never dry. The humidity term introduces error as the plant humidity decreases. That is, unless the moisture content is measured and inserted into the calculation. You’d need to use a moisture balance or some other technique.
In the end, it doesn’t really matter. Either way, you are setting the humidity and temperature (or VPD) to a range that is effective and safe for drying … and keeping the range balanced through the drying cycle. You’d end up in the same place either way. 