If you want to export multiple VST Instrument channels (multi-timbral) that have their MIDI information on MIDI tracks:
- Drag each OUTPUT of the VST instrument track up to the top of the screen.
- Select the top VSTi channel (this will export first).
- As the export progresses it will then solo each VSTi channel AND the appropriate MIDI channel, moving downwards as usual.
Just be sure that each track you want to export is next DOWN.
See screenshots below for more detail: