Yes Vista 3 will run on 10.12 to 10.15. We are even confident it will run on the upcoming 11.0, based on our beta testing of Big Sur.
If you stick with Vista 2 and later update your Macs, we wont be able to guarantee Vista 2 will work, since it will never have never been designed to run on those operating systems.
You can’t have Vista 2 and Vista 3 installed on the same Mac at the same time, but you could uninstall 3 and install 2 again if you really wanted to. Having said this, I am confident that once you start working with 3 you won’t want or need to go back. 3 is a million times better than 2!
Vista 3 will work with all of your Vista 2 showfiles. It will even ask you if you want to “auto import them” when you first run Vista 3. It’s as simple as that. Install Vista 3 - open your showfiles, just like you would in 2.
One thing to be aware of is showfiles created (or saved in) 3 CAN NOT then be re opened in 2.
So it’s a good idea to always have an external backup of your 2 showfiles, then you can rest easy knowing that everything is safe and you will never loose anything.