Here is a link to an ISO used by many members.
With this ISO, you can burn a CD with Vista.
You can then use that CD to install Vista as many times as needed.
Dell Community
Many members realize that an ISO is not the best way to go, however.
Everyone, from time to time should use the free Macrium Reflect to create an image of their Hard Drive and keep it on an an external hard drive.
Macrium Reflect FREE Edition - Information and download
In the event of a problem, you have the current image and you will not be required to reinstall software, etc, as will be necessary with and ISO. In addition with an ISO, you must use a Valid Product Key number.
With an Image if you make an image of an OS that is installed with a proper key number, there is no need to reenter the number. In addition, the image will be current all updates, etc, which is not the case with an ISO.