Hi Jim,
While I can't assist you directly with your request for someone to do a mockup of your piece, I can suggest that while that approach may give you a sense of what is possible with VSL instruments, you will ultimately be at the mercy of the competence/effort of the individual that you have hired. Working with midi, especially to realize a piece with all the nuances and aesthetic considerations you likely would expect (based on your suggestion of a string quartet) requires an inordinate number of hours, which may make it less feasible, unless you find someone willing to do it for the sheer pleasure, and not for adequate compensation.
The other concern is, that even if you are satisfied with the individual's rendition of your piece, it doesn't do anything to help you with your own efforts. That will require a fair amount of practice, experimentation, and oodles of patience. Remember, your musical skills/composition chops have no bearing on your ability to feel comfortable working with daws, virtual instruments, plugins, mixing, and mastering. Most of us that pursue this, coming first from a musical background, suffer years of frustration as we hone our skills. There are amazing composers that wouldn't have the skill/patience/desire to ever render their pieces with virtual instruments, and there are less accomplished composers that possess a lot more skill in terms of computer music production chops.
With all that being said, my suggestion to you is to definitely try to find someone to commission (trust me, there are many composers that will welcome a pay day doing what they love, even if it is less than an ideal amount), but to also demo a few of the virtual instruments offered by VSL and try your hand at it yourself. I believe you can demo them for 30 days (could be wrong on this?) so while an "expert" is working on your piece, you can at least explore the articulations, and workflow with your preferred DAW.
Hope this helps, and best of luck!
Dave
p.s. if you do get your piece done by someone, please share it here, along with your own thoughts on how it turned out!