Having said all of this, the measurement of a PCB plane using a VNA may not be as straightforward as you might expect. One simple trick makes it easy.
A 2-sided 63mil FR4 PCB is used as an example in this article. The PCB is measured using both a VNA and a TDR. The results are compared, the error source is identified, and the trick is employed to correct the error.
The PCB is shown in Figure 1 connected to a SD-24 20GHz/17.5ps sampling head installed in a Tektronix 11801B DSO mainframe. The board is connected to the instrument via an SMA edge connector and a SMA male-SMA-male barrel adapter. The instrument cursor table is enlarged and shows a characteristic impedance of approximately 3.36 ohms or 10.4dB ohms. I say approximately because the cursor resolution on this instrument is somewhat coarse, but it is pretty close to 3.36 ohms.