A VE instance can be saved with every MIR setting, and all the instruments can be removed or replaced but the MIR hall settings will be the same.
I know. However, it's helpful in certain cases to be able to save the MIR "room" settings independently. In my case, I have a Strings Instance, Woodwinds Instance, etc. but I want them all to be in the same room with the same settings. If I want to change any of the room settings and have them applied to all instances, all I have to do is save the new MIR Engine settings and load them in each of the instances. If I saved it as a VE instance project, I'd have to reload each instance's instruments to their respective section (Strings, Winds, whatever) each time I wanted to change a parameter.