VSTi's are handeled like VST-plugins, they are just listed (and selected) differently. It's on p. 33 of the MIR manual:
"Preferences > Plug-ins: Shows a list of VST (not VSTi) plug-ins available to MIR. Clicking on
the VST Settings … button allows you to browse attached hard disks for folders
with installed plug-ins [VSTi's too!] and add them to the list. Alternatively, you may remove
folders from the list, or rescan selected folders for recently added plug-ins.
IMPORTANT! Only VST effect plug-ins are shown in this list. All valid 3rd party
VSTi’s (VST instrument plugins) appear in the “Source”-pulldown of the “Add
Instrument”-window (see section C 2.1)
CAUTION: Only 64-bit compatible VST2 plug-ins will do! Be careful when adding new plug-ins to the list, as there always is the
possibility of some unforeseen conflict halting the whole system.
HINT: With the help of so-called VST wrappers like jBridge7 it is principally possible to use 32-bit plug-ins in a 64-bit environment
like MIR. In many cases, this will work wonderfully, and significantly increase the number of possible audio plug-ins. However, we
hope you will understand that we do not (cannot) actively support this special workaround."
HTH,