MIR (Multi Impulse Response) is the concept.
MIR Pro is the most recent, full blown product based on that concept. It can be used either as full-featured part of Vienna Ensemble / Vienna Ensemble Pro, or as a more conventional VST/AU/AAX plug-in with a slightly restricted feature set (but the same sonic possibilities).
MIR Pro can be thought of as the "sample player", while the so-called RoomPacks encompass the "samples", i.e. the collected impulse responses from selected stages and studios (the Venues). A single Venue can contain up to 6,000 IRs. Each RoomPack relies on an individual license which is valid for all included Venues.
MIRx is a spin-off product based on highly elaborated factory settings derived from six (or actually seven) selected MIR Pro Venues, designed to be used directly (and exclusively) within Vienna Instruments / Vienna Instruments Pro software player. For maximum ease-of-use it offers hand-made parametrisation for more or less each Vienna Instrument available. MIRx is sonically identical with MIR Pro, but it can't be edited, and the two products are not compatible data-wise and rely on different licenses.
To grant the users of MIR Pro access to the aforementioned thousands of hand-made presets, a MIRx Mode has been added as a special functional layer to MIR Pro. Provided that there's a license for the respective Venue, MIRx settings can be used and adapted freely within MIR Pro.
Please don't hesitate to ask if there are questions remaining. :-)