Technically, you could make a chart with either leaf or room temperature on the Y (left vertical) axis - so it's a good question.
Here's a
chart where they actually label the axis as "Room Temp."
For the Pulse charts, I worked through the math (
here) and compared to their example charts (
here). Pretty sure they're using room temp on the vertical axis.
(For RH = 0%, Tair = 102F, Tleaf = 100F - I get 6.49 kPa, which corresponds to 102F / 0% RH on their chart.)
To clarify, there is only one VPD in the tent and only one target value (for each growth stage). The VPD (vapor pressure differential) is the difference in air SVP (saturation vapor pressure at the room temp and room RH) and the leaf SVP (saturation vapor pressure at the leaf temp and RH = 100% inside the leaf). The question is really: which temp (air or leaf) is whoever made the chart listing on the chart axis when they report VPD values in their chart.
And, you're right - most articles / calculators don't take the time to clarify. In general, I would assume room temp, but you can always work the math yourself to confirm.