Microsoft has released a new DirectX redistributable package into the wild for virtually all versions of Windows, including Vista. This is a DX9.0c / DX10 update that's not going to show up on the Windows Update website, but it is an official release.
"This download provides the DirectX end-user multi-languaged redistributable that developers can include with their product. The DirectX redist installation includes all the latest and previous released DirectX runtime. This includes the bi-monthly D3DX, XInput, and Managed DirectX components."
This is not going to make your Vista framerates jump all that much, if at all, but with the relative immaturity of Vista and graphics drivers for Vista, we recommend downloading and installing this package to bring your OS up to date.