By default Microsoft does not allow you to install third party themes on Vista.But after using this trick you will be able to use third party themes in Vista.
To install new themes in Vista you have to patch the windows system files first. To do this log in to admin account in your Vista.Download VistaGlazz from the link given here.
Unzip the downloaded file and run the app.
Click on “I agree” then click on the first Icon in the window.It will patch your system and system will reboot.
Now move your theme files to Windows\Resources\Themes.Go to desktop, right click there and select Personalize and then Themes. Select your new theme from the drop down list, click apply and enjoy your new theme.