B3D stuff does indeed work on Vista. B3D may be DX7, but some people seem to be forgetting that DX9 is backgrounds compatible ... it's DX10 that isn't!
Ok, after reading your post and then re-reading the Wikipedia info, I think I get it now.
I think you're saying that the "legacy version of DirectX 9.0c" will support DirectX 6, 7, 8 etc. - but DX10 no longer maintains seperate "interfaces" for older versions.
If I'm understanding that correctly then great - B3D and lots of other stuff should still run, maybe just requiring a few tweaks here and there as you said.