The Sonar Reference Manual goes into great detail about instrument definitions, but:
 
You  Can  Not  Make  An   Instrument   Definition   For    A   VST  Instrument.  
 
It would be a really really good thing to be able to do.   The reason you can't apply an instrument to a vst is because the VST midi port is temporary ... but why not, once the VST is loaded and assigned to a (new, per project) midi reference, allow an previously created instrument definition be applied to it?    Seems like a simple and useful improvement.    I use the same 127 violin patches over and over, and have to mentally keep track of the patches based on number, while ideally I could have a defined instrument that says:  "#47:   Tremolo, crescendo p to mp, 3 seconds"
 
The reference manual says it just right, page 1394:   "Why use Instrument Definitions"