What determines the longest group delay a VNA can measure?
Essentially, it is the spacing of the frequency list. the frequency spacing must be less than 180 degrees between measurement points. Smaller steps means longer delay. The delay is delta-phase/(360*delta-frequency). 1 MHz spacing is good to 500nsec; 1kHz is good to 500 usec.
But The noise on the phase is essentially fixed (say 0.001 degree noise), so the noise on 1 kHz spacing is 27 nsec; the noise on 1 MHz spacing is 27 psec.
Retrieving data ...