Is X3 compatible with Vienna Ensemble Pro 5?  
I would like to make use of a spare PC I have, in order to off-load some of my virtual instruments from my DAW to save CPU.
It looks like my best solution might be a virtual instrument server such as Vienna Ensemble.
Apparently it is designed to work along side most DAWs, but i thought i better check before i go ahead and get it. 
Thanks for your time.

There's absolutely nothing to suggest that Sonar will have a problem running Vienna.
 
Depending on the power of your main computer, I wouldn't even think about offloading Vienna to a separate machine.
 
I run EWQLSO Platinum on my machine with zero problems (and about 135 tracks)