When you are trying to upgrade the system to Windows Vista, and the profiles are outside the system drive (example, profiles are in Drive D:), you will see the Upgrade option is grayed out, only ‘Clean Install’ is available.
This behavior is expected and by design,
Vista upgrade is not designed to support profiles outside the system drive. At this moment, you need to use “USMT and Easy File Transfer” to migrate your system to Windows Vista
Zero Touch Installation (ZTI):
Center - BDD 2007: