You know, it's also possible that he got into an accident, and decided to just park it in a lot and walk away. Also possible he got lucky and is in a nearby motel room with your VX parked out front! In any case, is the VX insured against theft?
Just be thankful he didn't kill someone with it while test driving it, becauese you would be responsbile!
** WORD TO THE WISE **
If someone is interested in buying your car, do not let them test drive it! Even if their intentions are good, you assume civil liability for any damage they might accidentally cause during the test - including personal injury or death! Why would you put your financial future at risk in the hands of someone you don't know?!
My own tale goes like this: I had an S2000 that I loaned to a coworker so he could take a girl to lunch (he drove a minivan). The roads were wet, and they took a corner too fast, lost control, spun out, and hit a lightpole which fell on top of the car and hit his head. With that, I had significant damage to the car, a broken lightpole, and his medical bills to pay - totaling more than $20,000. Fortunately, my insurance covered it, but I'm still paying elevated rates because of it.