So why not just use it with 50-ohm center on the Smith Chart? It will easily show 75-ohms to the right of center on the central horizontal line with a non-reactive resistor. By design, the NANOVNAs are 50-ohm devices at both ports as are high-$$$ units. Pretty much only the TV and CATV people use 75-ohms. The rest of the RF and µW industries use 50-ohms.
If you absolutely require 75-ohm ports, you could construct a loss matching pad from 50-ohms to 75-ohms. You will lose a bit of dynamic range in doing so.
