Install DirectX 9.0c on Vista. Preferably the latest update. This will add the required Dx9 DLLs to the system and allow you to play the games flawlessly.
Note: Doing this will not affect your Dx10 status at all. The installer will automatically know what to do and just fill in DLLs and not replace Dx10 with 9. So its safe, do it without worries as this is how we've all been playing Dx9 games on Vista.