Dave, once again a very instructive and well done video on low value current sources.
The VHP100 donated by Vishay is even more stable, than you thought. They are real jewels..
First, this type is hermetically sealed and oil filled!
That removes any oxygen and humidty, and delivers an ultra stable shelf-life (no or small load) over time, as small as 2ppm/ 6years.
The non hermetically sealed ones, i.e. the molded versions, age around 20ppm/year, also shelf-life.
Second, the VHP100 resistors are even better regarding T.C. than the Z foil types.
The Z foil have 2ppm/K max only.
The VHP100 contain 2 resistor chips, one of the C-type, the 2nd of the K-type bulk metal foil resistors.
Those were the denotations of the older resistor technologies from Vishay.
K and C type have opposite sign of T.C., and are matched in this package, to give a much better overall T.C. compensation.
Therefore, the VHP100 have max. 10ppm change over 15..45°C, that is a max. T.C. of 0.3ppm/K.
Therefore, treat those resistors very carefully, they can serve you as real ultra stable resistor standards.
You only have to determine their exact values once, and then keep them away from loads > 10mW.
Best you assemble them in a case with 4 terminal Kelvin connection for best accuracy.
Frank.