The Dell Windows Vista Reinstallation DVD applies Dell OEM SLP activation (offline BIOS based product activation) so yes you can use it to reinstall Windows Vista. However at this stage there is little point in installing Windows Vista as it reaches End of Life in April. This will make it more insecure as it'll have no security updates. Already all versions below IE11 are no longer supported and IE9 is the last version of IE that runs on Vista. Google Chrome is also not supported on it.
I have a Dell XPS 420 that I use from time to time. A few months ago I booted it up and got a message that the KEY was out of date (ran out) and I would like to know how to get a new one. I have a reinstall disk that came with the computer when I bought it new from Dell. There is a key number on it .At least I think its the key. Can I  reinstall Vista and use this key to get my system back up and working? If so are there any problems I should look for? If not, how do I get a new key for the one that's already installed?
Vista Home Premium Key