Hi Phil,
what you have in Vienna Suite's Convolution Reverb is more or less what you would expect from a professional convolution reverb today: Conventional stereo (2-chan.) or dual stereo (4-chan.) impulse responses from one single perspective of a hall.
For comparison, this is like a single velocity of the middle C of a Boesendorfer Concert Grand, recorded with one microphone. If MIR would be about pianos, it would give you all 88 keys in a multitude of velocities, so to speak, recorded from a multitude of angles and distances.
MIRx for Vienna Instruments and Vienna Instruments Pro is derived directly from MIR Pro, but you neither can change the pre-defined positions of the instruments on a stage, nor can you change the (virtual) main microphone's position which "records" them.
In other words: Conventional IR-based reverbs are like nice black-and-white photographies of a hall. MIR Pro gives you the virtual-reality "holo-deck" model of that hall. MIRx is a colorful 3D-movie with a fixed story line. ;-)
Kind regards,
/Dietz - Vienna Symphonic Library