You don't have to buy any MIRx Venues if you want to use MIR Pro - just download the update for MIR Pro from your "MyVSL" user-area, and you'll see the new presets in the MIRx-Mode panel. This is true for MIR Pro as well as MIR 24.
MIRx (opposed to the aforementioned "MIRx Mode") is an product by itself, it relies on different data and different licenses.
... see this older thread for a comparison of the two: -> https://www.vsl.co.at/community/posts/m224364-MIR-PRO-v-s--MIR-X#post224364
HTH,
/Dietz - Vienna Symphonic Library