in theory, when u upgraded to 7 you should have made a system restore. If so, then you can go back to the system restore and everything will be normal...
If not, then you need to back up all your data and reinstall Vista
As much as I like Vista, I don't think I'll ever see the need to go back to it once Windows 7 is installed. However, it should be possible as long as you retain the Vista OEM product key and media.