The goal of a VSA is to demodulate a signal and analyze the spectral content in time domain, this is why it needs to digitize the signal BW in real time and use the right demodulator to analyze it. The fact that both can show the spectrum of a signal is irrelevant in this context. One is a Sprectum analyzer and the other on is a Vector Signal analyzer. These a 2 different instruments for 2 different types of measurements.
