VPD relies on two things. Temperature and relative humidity difference between two things.
If the relative humidity is different at a different altitude, then the VPD will also differ. If on the otherhand RH and temperatures are the same, then there should be no change in the VPD. Air pressure is not considered.
See these equations:
Vapor pressure is calculated by RH * saturation pressure. Vapor pressure deficit is the saturation pressure of the plant matter subtracted from the saturation pressure of the surrounding atmosphere. Plant matter is considered 100% saturated. Saturation pressure relies on temperature.
The variable that would affect VPD is the RH and temperature.
This is not saying the effect on transpiration (and subsequent uptake) is not occurring at altitude. What it does say is that perhaps we are looking at the wrong thing. Maybe the measurement of RH is problematic. Maybe the stomata close down for some reason. Maybe the leaf temperature differs (who measures that?). idk. Interesting though. Dew point, on the otherhand, …
Dive deep: Humidity_Handbook.pdf (2.8 MB)
Also, see
https://wat.lewiscollard.com/archive/www.newton.dep.anl.gov/askasci/wea00/wea00282.htm