directx 10.1 shipped w/ Vista SP1. So you have it on your computer.
If you Have 3DMarkVantage, and you use 'View System Info' it tells you which version of Directx you have. I have a Nvidia GPU, and it tells me that i have Directx 10.1 . So even though my hardware isn't 10.1 compliant, i still have the software.