Yes, there are some more complicated methods, but I told you about the simplest one. The best is to search Agilent Application Notes. There you may find detailed description how to use VNA with power amplifier. Calibration procedure with attenuators is not absolutely perfect because it change the directivity for ports, but usually it provide good results and it is practically the same as standard SOLT calibration. The only difference is that now attenuator or attenuators included bee tween port(s) and calibration planes.