Why A vector network analyzer sends a known signal into a port and measures both the magnitude and the phase of the waves reflected back and transmitted through, giving the full set of S parameters. S11 and S22 are the input and output reflection coefficients, and since reflection coefficient and impedance are directly related (Z = Z0(1+Γ)/(1-Γ)), the VNA can display input impedance, output impedance, or Γ itself from the same measurement. S21 and S12 then give forward and reverse gain or loss, so essentially all the port characteristics of a network come from one sweep.
Watch out Picking just one item misses that all three are the same data presented differently; impedance is simply the reflection coefficient converted through the reference impedance, usually 50 ohms.
Vector = magnitude AND phase, so a VNA gets Γ and every impedance that follows from it.