If you're like me, there's a good chance you're going to want to have Service Bulletin #12. This problem is one where it will fail calibration at boot sometimes or maybe all the time if it's really bad. (mine was intermittent). The story I read about this problem is that it plagued Ensoniq's engineers for a couple years almost. And it also affected more than one machine. I"ve even seen a document that had handwritten notes from the engineers talking to one another about the problem. They tried swapping out practically everything before they finally figured it out.
Basic gist is this… the VFX-SD has an achilles heal in its design. Namely, there are three circuitboards under the keybed that are tied together. Over time, the keybed being pressed causes this area to flex where the circuitboards are. This flexing eventually causes the ribbon cable connecting two of those boards together to fail. This will usually be seen in keyboard calibration failing at boot. But other times, it will boot just fine and even work for awhile; but as you play, the flex causes it to crash or do other weird things. It's a very common problem on VFX-SDs (and a couple other models if memory serves). If you have this problem, the good news is that the fix is pretty simple (albeit somewhat cumbersome). You simply solder new wires in place (or even in addition to) the existing ribbon cable. That's it!
Anyway, this information is hard to find despite being common. Here's a link to where you can view it. http://imgur.com/xVVXW0d