using nero express and the bottom option (nero 6, might be different in 7, i dont know) "disc Image or saved project", choose image files in the "files of type" filter box, browse to the vista ISO and double click, click next to start the burn. that should be all you need to do.
if that fails, it might be that the iso file is corrupt. check it with ISO Buster or magic ISO, this will allow you to view inside the ISO and possibly burn the image to cd/dvd
i havent used iso buster but magic iso will show you the files within the iso (but you cant do anything with them, they still need burning).
it might also be that nero7 is corrupt, a uninstall/re-install might cure that.
best i can offer im afraid.