1 Load the VSTi.
Right-click in an empty area of the track control panel, select "Insert virtual instrument on new track".
The FX Browser will appear. Browse to your desired VSTi, double-click it to load it (I am using Kontakt 3, 8 outputs here).
... Big pic:
https://i.imgur.com/QvCTiiq.png
Reaper will report that it is about to build the output audio tracks for this VSTi; click [OK].
... Big pic:
https://i.imgur.com/TZqdWLL.png
2 You now have a single MIDI input track for the VSTi and the VSTi's output audio tracks.
... Big pic:
https://i.imgur.com/NymCe7l.png
3 Here is the Routing panel (click [io] on the VSTi track):
- you will see that Master/Parent Send is unticked, so any audio is not sent to the Master Mixer,
- and that the channel pairs are sent to the 4 audio tracks.
... Big pic:
https://i.imgur.com/6iMnd7j.png
4 All you have to do now is load the Instruments in the VSTi; in Kontakt 3:
- load each Instrument,
- set its input MIDI Channel, if needed,
- create the additional Kontakt Mixer channels if needed,
- set its Output to the desired Kontakt 3 Mixer channel,
- configure the Kontakt 3 Mixer, if needed, routing its output channels to the Reaper track's channels.
5 Here I am playing 2 instruments in Kontakt 3 with their audio outputs routed to Reaper tracks 2 and 4.
... Big pic:
https://i.imgur.com/uYYG3lR.png