Its very simple why the Framerate in Vista is much lower. This is because of the new programmed DirectX 10, so DirectX 9 doesn't fully work anymore. I tested it with Battlefield 2142 and when I first started the game, I got an error that said something about a DirectX 9 DLL file. This is because the DirectX 9 Api's (DLL's) are not included with DirectX 10. So If you want to play DirectX 9 games, you must install the latest DirectX 9 version.

Only the problem is that there is no real support for DirectX 9 anymore, so the DirectX 9 files are being Emulated by DirectX 10 which causes a much lower Framerate then in Windows XP with the REAL DirectX 9. So If you want to play DirectX 9 games at full speed under Vista, you'll need a fast graphics card (like an DirectX 10 GeForce 8800 card)
If you don't have enough money to buy one (like me), there's only 1 option -> Stick with Windows XP and there will be no problems!