Hi rissa85225, welcome to TSF.
DirectX 9 and 10 are mutually exclusive. in other words, they don't rely on each other to run and both can be installed on a system without creating any conflicts.
The version for DirectX 9 that comes with Vista does have quite a few issues so it is always best to install the latest version of DirectX 9.0c.
You can get the latest version from
here.