Microsoft has released a new DirectX redistributable package into the wild for virtually all versions of Windows, including 64-bit versions of Vista and XP. This is a DirectX9 and 10 end-user runtime update that's not going to show up on the Windows Update website. It is an official release, however.
"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 its graphics drivers, we recommend downloading and installing this package to bring your OS up to date.