1. Open your Windows 7 drive and make sure that you have Windows.old in the root of the drive. We use Windows.old folder to restore Windows Vista from Windows 7. So without Windows.old folder you can't downgrade to Vista from Windows 7.
2. Next make sure that you have enough free space on your Windows 7 drive (generally "C" drive) as your need to move all the contents of Windows 7 to a folder and then need to copy Vista contents from the Windows.old folder.
Jot down your DVD drive letter and also Windows 7 installed drive letter.
3. Insert your Windows 7 installation disc in the DVD drive and reboot your PC to boot into the recovery environment.
4. In the first screen, select your language, time and currency format and keyboard input method and click Next.
5. In the Install Windows window, click Repair your computer option to launch System Recovery Options dialog-box.
6. Select your Window 7 OS and click Next.
7. Here click Command Prompt to launch the same.
8. Now type following commands one by one and hit enter after each command to move all the contents of Windows 7 to a new folder (here "C" is your Windows 7 drive letter).
Move Windows Win7\Windows
Move "Program Files" "Win7\Program Files"
Move Users Win7\Users
Attrib –h –s –r ProgramData
Move ProgramData Win7\ProgramData
Rd "Documents and Settings"
9. In this step, you will copy the contents from Windows.old folder to the drive. Now type following commands one by one and hit enter after each command (here "C" is your Windows 7 drive letter & D is your DVD drive letter).
move /y c:\Windows.old\Windows c:\
move /y "c:\Windows.old\Program Files" c:\
move /y c:\Windows.old\ProgramData c:\
move /y c:\Windows.old\Users c:\
move /y "c:\Windows.old\Documents and Settings" c:\
10. You are almost done! It's time to restore the boot sector of your Vista OS by entering the command given below (again, "D" is your DVD drive letter):
D:\boot\bootsect /nt60 c:
11. Finally type exit and then reboot your PC to start using your Windows Vista OS. You can now safely delete the Win7 folder in the root of your "C" drive to free up disk space. Good luck!