Well, the short answer is: too slowly. Max frame rate I could get was about 6 FPS. I tried all the tricks I know from when I had an XP machine; i.e. doing ctrl-alt-delete (right alt-delete in a virtual machine) to close a lot of running apps in the background, lowering screen resolution, etc. etc. but nothing seemed to help. And 5-6 FPS is simply too painful to watch. I've loaded many other games on virtual machines I've created and all have worked beautifully with my Nvidia 8800GT card and my quad-core Dell XPS ... but not BVE. I'm really at a loss here other than to say I feel the culprit(s) are the Nvidea drivers. So it's back to trying to get BVE to behave under Vista.
I was able to load BVE4 normally (i.e. *not* in a virtual machine but rather in my host Vista operating system) after having registered those dll files in my System32 directory; oddly enough when it loads you can see a bunch of graphics loading that you normally *don't* see when you are running BVE4 on an XP machine running something other than an Nvidia card. Only when the route is fully loaded does the screen go blank! Speed in loading is just fine (unlike the molasses-like virtual machine XP install), and the sounds come through loud and clear. I loaded the Birmingham X-city route, which I like a lot, and was actually able to drive the train just by sound a fair distance! But that's obviously not a real solution, and I'm afraid right now the only other option is to go out and buy an ATI card.
Hmm, I think I'll just wait for OpenBVE to become fully developed!
(BTW, the version of OpenBVE out right now, 0.8.9.3, runs the above route quite nicely - if not as visually pleasing as in regular BVE4 - so I have high hopes for this solution.)