Why A vector network analyzer sweeps a known signal into one port and measures the magnitude and phase of what is reflected and what comes out the other port, giving the S parameters S11, S21, S12 and S22 versus frequency. S21 is exactly the transmission response of a two-port device, so a filter's insertion loss and passband shape across frequency fall right out of the measurement. The 'vector' part means phase is captured too, so group delay and complex impedance come along with it.
Watch out Phase noise and pulse rise time are time-domain or noise-spectrum measurements that call for a spectrum analyzer or oscilloscope, and forward power in a live transmitter line is a wattmeter job; a VNA measures ratios using its own low-level test signal, not a transmitter's output power.
VNA = S parameters = response versus frequency. S21 is the filter's frequency response.