Umm yeah this should have been known. Sorry but Vista has had DX10 since its early beta stages. If you need DX9 to run a game you have to download and install the DirectX Runtimes. Also can download the SDK. Which the latest can be found here...
Microsoft Download Center: DirectX
Just get the newest package and install. It wont overwrite your DX10 install and still allow for DX9 games to be played.