Dietz,
Unfortunately I have no experience with Omnisphere or Kontakt eiither so that comparison is lost on me. I am not sure what Sonar uses that would be analogous to an "instrument rack" - I'll have to check into that. The way I have done things in the past is to assign a track (in the case of Sonar the VST goes into the FX bin of the audio track and you setup a coupled midi track for midi info), This is the only way I know of to create separate tracks - one for each instrument. If all instruments are loaded into VE and then send to the DAW, how do you record instruments separately?
Mike