And just now, after your recent return to Bz, when you were about to get in form and reach your old playing field... Damn new PC's!

I hope you haven't trashed old one... Many have spare PC just for Bz.
My guess is your problem is not in Vista, it's in Nvidia 260. That's the newest series of the cards in the market. As far as I know no one in Bz has it. People with promblems have series 8 or 9. Your card came after those, it's a brand new, so there are no older drivers. Just make sure you have the lastest ones (181.20 - released January 8, 2009.). If you do, there's nothing more you can do with your card but to wait for the next drivers release (every month new driver) and hope that those will work better.
Those 1,2,3... points to follow for Vista/DX10 are not a guarantee for a fix, because as I already mentioned, there's no general Vista/DX10 fix yet. For every PC is a different story. That's why I hoped Dark Oddity's suggestion of using MS Compatibility tool will work. Those points (1,2,3...) are things I collected from other posts and even other forums, which were helpful to other players with the same problems, so I put them into same place (same post) to be easier to find. One, with the problems, should do a single change then test, then if it wasn't helpful undo the change, then the same procedure with the next point. And, of curse, notify us what was helpful.
Now, Indeo codec is missing in Vista and it's needed for Bz video files to run. I have original Bz CD (Xplosiv) with 2 extra programs on it (Abobe reader for PDF guide and Indeo codec for videos). Also reaper's BzE mod is coming with that codec too. What I want to say, that codec can't hurt you, but you can try to uninstall it, to be sure.
As for compatibility mode, Bz is written in Windows NT 4.0 (between win95 and win98). Win98 are the best Windows from that era, and it is confirmed that some things in Bz won't work (few single player mission intros, like Lander scene in the 1st) without setting the compatibility mode on the application after the install (any of those 3 mentioned windows, but I suggest win98).
You tried all that and it didn't work. Besides waiting for the new graphic card driver, I suggest you to turn 3d card off in Bz options and play in software mode. Won't be the same look, but hat should work for you.