Add one plugin channel for Kontakt in Vienna Ensemble, set the midi channel to ALL.
Click the "+" sign at the bottom of this channel's fader to add subsequent instrument channels.
Make sure your instruments are assigned different midi channels within Kontakt. Set output routing in Kontakt for one stereo channel per instrument. Set the first instrument to midi channel 1, output 1, the next instrument midi channel 2 output 2 etc.
Hope my memory got this right, if not:
https://www.vsl.info/en/manuals/vep7/multi-timbral-plugins