Hi,
I remember a discussion on this same matter, but I can't find it anymore. However: MIR can be used in a VEPRO instance as an integrated effect, or as an ordinary plugin. In the first case, you can only see the icons of the instruments in the current VEPRO instance in a MIR window. In the second case, the MIR window contains all the icons of all the instruments in all the instances.
Now, unless I’m wrong (not at my studio, and the eLicenser is not working, so I can't try from remote), choosing a room in one of the instances also changes it in the other instances. Various global parameters should be mirrored in the other instances. Therefore, you don't risk to have several different rooms in MIR at the same time.
Also, you will be able to open two VEPRO instances into two separate windows, so it is quite easy to avoid overlapping between instruments, when moving the icons around in the different instances.
With dense arrangements, having separate MIR instances can be useful, to make the editing pane cleaner. For example: if you have the Orchestral, Chamber and Dimension Strings sharing the same space, it is easier to access them separately than in the same window.
Paolo