You have to understand that technically moving to Vista is not an upgrade in the sense that past editions were. Vista is installed using an image file (.WIM) by design. The "work around" is simply how that can be managed.
Microsoft calls this process a migration, not an upgrade. There are no common files from previous versions that are retained as in the old .CAB days.
Further, all of the packaged DVDs are identical - regardless of what version you buy. All the versions are present on the DVD. What makes it install one vs. another is the key ID used.