Bill,
There is some confusion going on here. The 2020 ADS-B rule states that we must have ADS-B Out in addition to either Mode C or Mode S. The technical term is ADS-B is "Additional equipage" to Mode C or S.
With that said, the Vision-Pro or any other ADS-B out does not replace your current Mode C or S transponder, with the exception of the 1090ES add-on to a Mode S transponder. That only covers the ADS-B Out and does not give you any receiving benefits to see the traffic every is sending our the traffic.
The Vision-Pro transmits on the 978UAT (ADS-B Out for GA under FL18). This out signal include the call sign and the ICAO code for your Mode C or S transponder. The Vision-Pro does not provide ES data, it's 978UAT, it does receive the 1090ES from other aircraft.
Mode S and ADS-B is two different systems. Mode S is non-GPS positioning and ADS-B Out is GPS position reported. Come 2020, ADS-B will be the primary and Mode S will essentially be the backup and international standard. You would need to enter the Squawk on the Mode C or S and on the ADS-B Vision-Pro. Some other devices support a Single Point of entry, which is an option that SkyGuardTWX is looking into as well. For the time being it does require a simple 2 point of entry to match codes. I hope this helps.