It's fixed! The issue was shockingly simple to resolve the final bit. I took a look at the black/yellow vacuum hose and followed it. Sure enough, it wasn't actually connected to the vacuum switch under the drivers side of the dashboard and near the center section. I have now connected it, fired the car up, and every vent selector seems to work. Thanks a ton for the help and for reference here are some photos of the vacuum line and the connection it needed.
I can't believe the final issue was this simple to resolve lol.