The Low Frequency Analyzer (LFA sometimes FRA for Frequency Response Analyzer) is generally used to measure control loops has an oscillator and two high impedance inputs .  The typical measurement is CH2/CH1 and ranges from 1Hz or below to 40MHz.  A few can go below 1Hz.

The VNA generally has pairs of 50Ω ports and measures (S)catter parameters as gain, impedance, admittance or reflection.

The LFA/FRA generally has significantly better dynamic range and noise floor than a VNA, though the VNA generally offers higher frequency capability.

The VNA typically offers SOLT calibration to de-embed the cables and probes while the LFA/FRA generally does not.

The VNA is generally more expensive than the LFA/FRA.