- Any VNA requires calibration, that's why any VNA owner also has a calibration kit.

- Yes, cheaper scopes (including DS1054Z), will need feed-through 50 ohms BNC terminators. Feed-through terminators are common items in any RF drawer, on par with 50 ohms cables. At low RF frequency, very easy to DIY.

- Amplitude and phase can be calculated with much greater precision than looking at the waveform on the screen, by downloading a longer waveform and post-processing it in the PC. Thousands of samples can be used for oversampling, in order to increase the ADC resolution, and DSP techniques over so many samples can get a very precise phase shift measurement. Repeat this for each Smith chart's point, and we can get decent performance.

The only major drawback I see is the speed. Downloading the data can take many seconds for each corresponding point on the final Smith chart, but that is strongly dependent of the requested number of points. Not all measurements need to have 3 decimal places.

Alternatively, the speed can be greatly improved considering most of the generators have sweeping frequency capabilities. So, instead of point by point measuring at each frequency, a frequency sweep can be made at once, and the full sweep response can be memorized by the scope then transferred later to the PC, by a single SCPI transfer for the whole frequency sweep. From there, the whole batch of samples can be DSP-ed together in the PC, in order to find the complex impedance evolution over time, and plot that on the chart. Sweeping like that could make a Smith chart almost in real time, or at least reasonably fast to allow "live" tuning of the DUT.

Trying to implement that will be a great learning experience, and this is the whole point. This idea is not meant to be a commercial product, it is not for the money, it's just for the fun of it, as a hobby.

You all just envy for when everybody with a DS1054Z will be able to do VNA measurements.