According to several forums, users have found that they could install upgrade versions of Vista without a prior installation of Windows XP. But be warned that this workaround runs afoul of the end user license.
The trick, its seems, is that one can install a 30-day trial on the upgrade DVD, then reinstall the fully-registered Vista off the same disc. This bypasses the need to have a valid copy of Windows XP pre-installed in your system before using the upgrade CD. Hence, this method makes the upgrade edition behave exactly like the full retail pack, with a savings of almost 40 percent.
However, this runs afoul of acceptable use of the product. When contacted by CNET Asia, a Microsoft spokesperson emailed: "We are well aware of the workaround. People without a licensed copy of XP that use this workaround are violating the terms of use agreed to when they purchased the upgrade version of Windows Vista."
Monetary savings or peace of mind? The choice is yours.