thats a different case, i did not measure before but
maybe signal source, amp, att, coupler to measure incident and reflected power , power meter might help
LO Return Loss is not a very significant measure.Because LO port is a "hard drivien" port and this LO signal will drive your mixer-either diode or transistor- to switch the commutating components and Retrun Loss has less importance.However, VNA's have generally +13dBm output power but if yours doesn't, simply amplify it but you can not measure accurately Return Loss because Output Return Loss of your amplifier will serve as your source and calibration wouldn't work since if you can find an ideal like amplifier.