impedence measurement with vna
Yes. VNA have special mode for mixer measurements. Check Agilent web site, they have good application notes for this measurements. But be careful with output impedance level. Some mixers have very high output impedance in several kOhm range. In such a case the accuracy of impedance measurement with 50 Ohm VNA may be low. Also you need to be careful with DC at the mixer output if it presented.
There is one more way. If you have S-parameter file for your mixer it is easy to simulate it with RF simulator and get the output impedance.