Yury Kuchura EU1KY
Consider the coax cable loss. The longer it is, the better SWR you observe. If you take 500 m of unterminated poor thin coax, e.g. RG-58, it will show SWR of 1.1. But all the power you put into it will be dissipated as heat on cable losses.
It does not matter how much power the VNA outputs, impedance measurement is independent of it.
SWR indicator in transceiver uses Bruene bridge with diode detectors. It demonstrates very low sensitivity when the bridge is close to balance, below SWR 1.5, and depends on power. This analyzer uses not a balanced bridge, but it measures voltage and current into the load, there is no deep null, and no detectors. This provides precision.