Hi Joe,
The Q you will measure is the operating Q and that probably is the one you want. That is to say, we have the Q on the plate side of the tube and the Q on the antenna side... that is the 50 ohm side and where you would place the VNA. So, with the pi net properly terminated on the tube plate side, use the vna and measure the return loss from its maximum best case, say 20 db until it drops to within 3 dB of its worse case. That total BW should be the operating BW of the pi net. Its value should be 1/2(Q_tube side+Q_ant side). So for example if the tube side Q were 12 and the antenna side 2, operating Q is 7. Usually the antenna side Q is quite small and the Q operating is set by the tube side Q.
