well ya learn something new everyday http://www.ngohq.com/news/16601-how-to-install-directx-11-in-windows-vista.html
"Apparently those who already own DirectX 11 hardware (ATI Radeon HD 5000 or better, Nvidia GeForce GTX 400 or better) don't need to upgrade to Windows 7 just to utilize DirectX 11. It is possible to install DirectX 11 on Windows Vista as well. All you have to do is to download and install Update for Windows Vista (KB971512). Be advised that Service Pack 2 and the latest version of DirectX Redistributable are both required."
Update for Windows Vista (KB971512)http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=3274