I just ran into the same problem you did. Was doing my usual thing of checking for program updates and updated VEPro 7 via the Vienna Assistant as it showed an update for VEPro 7. Nowheres was there any warning about needing a specific minimum of OSX and the update installed right over the old one. I too am in the same situation as you about going to a newer OSX and needing a new video card. What is needed is a Metal compatible card and new cards are going for around $500 to $600 US. Have the same Mac you have except for a single 3.46 GHz hex core cpu and 48 gig ram. Just not worth it for a 2012 computer although it's great otherwise. I have just the one computer.
Like you I luckily had an older version saved off to a separate drive; I save all my downloads so if I need them I can access them without having to go online. What I had to do was to uninstall the version that the updater installed and then install the older version.
What to do for the future and others who may be in the same situation? Keep all your downloads off to a separate drive. Make backups of your working system. The most important thing though is on Vienna's part and that is to have the updater say what minimum operating system is required and to not allow the install if a too old operating system is found. One more thing Vienna could do is to allow access to older versions but unfortunately that is not possible right now for the iLok protected software.