You want to make a bootable DVD to install Vista from?
you need a burning program which will burn an ISO image. These are free: http://www.imgburn.com/
You may already have such a program like Nero for example on your PC.
Once installed you should be able to 'open' the ISO file with imgburn and it should offer to burn the image to a DVD for you. It should be a very simple process with the right software.