Some modern VNAs have this Optional Feature. I cannot illustrate here but you can find lot of information about it for instance Keysight's website. Simply you drive the Mixer by LO and the difference (or sum) IF frequency is received by VNA and you see the Gain directly. Here, Off-set means that VNA while Port1 is sweeping the driven frequency, Port2 sweeps LO+RFin frequency so there is an off-set.
In fact, there is a wizard for this purpose and all you need is to enter the required values. VNA computes the rest.