Oh, Wow! An HVAC designer. So COOL. Pun intended :-). The bud should be at a higher temperature because the leaf stomata serve to regulate transpiration and thus lower the leaf temperature by the isothermal evaporation of water vapor.I would think that the surface temperature should be taken at the leaf. The VPD is the difference between the room conditions and leaf conditions. The bud is probably somewhere in between these conditions (is my guess). Here's a Calculator where adjusting temperature and/or humidity calculates the VPD.