windows vista isn't even 4 gb, if you have the whole dvd iso, it will fit on an ordinary dvd.
your mother board must support booting from usb first, then you set it as statedin the first post. however, this still wont work in this case, i'm pretty sure as you need to be sure to include the system files on the drive and i'm gonna guess you don't know what or where they are. just burn a genuine copy of vista on a dvd, or visit microsoft for a replace,ment media, they're about $14 +s&h and use the key you have, unless it;s an oem, then get it from the manufacturer of your pc/laptop, about the same cost, in my experience