OP: if you have the air temp and leaf temp by different sensors there maybe that the difference is not actually that large, two sensors may measure slightly different. When working with vpd i always think that its more an indication on which direction moving your climate, the actual indicator if all is well is in your plant stance/growth. Also leaf temp/air temp difference tend to be larger the less light you have on the cannopy. You can try your leaf temp sensor on different leaves, comparing corners to centre under the light.