Agilent 89600 is not a Vector Network Analyzer (VNA). Is a Vector Signal Analyzer (VSA).
I/Q gain imbalance results in an asymmetric constellation, and I/Q offset errors result in a constellation whose center is offset from the reference center.
The best way to verify most I/Q impairments is to magnify the scale of the constellation and look at the EVM metrics.
i noticed some datasheet plots a static I/Q mismatch data, e.g., the static phase error shows that 3sigma phase error is less than 0.5°, or something like that, how can they do that?