I have swapped my laptop for one which had Vegas 8 on before I got it. The previous Vegas8 is uninstalled but the old keys must still be in the registry because when I try to install a new version of Vegas it does not ask for a serial number.
You could use RegEdit and search for any key with "sony" and "vegas" in it and delete it. Might not be a bad idea to delete any key with just "sony" if you haven't installed any other Sony applications yet either.