Per the Windows Vista end-user license agreement, you aren't permitted to install the Home Basic or Home Premium editions of Windows Vista as the "guest" operating system in a virtual machine (VM). You may host other virtual machines on any edition of Windows Vista.
Additionally, one license of Windows Vista is exactly that -- one license. You can install a valid edition of Vista into a VM as a guest, but that counts as the installed machine; you can't later install Vista onto a physical machine, at least without first completely destroying the virtual instance. For the sake of completeness, I'll note that there is no technical restriction that enforces the kybosh on Home Basic and Home Premium as a guest VM operating system.
Windows Vista Enterprise has some tangible licensing advantages if virtualization is part of your strategy
This edition is only available to volume-licensing customers, and I'm sure this exception in its license is meant to be the carrot to the above point's stick.
According to Microsoft, you may install Windows Vista Enterprise in a virtual machine as the guest operating system. Not only that, but you can install -- under a single license -- one copy of Enterprise on a physical machine, and then up to four other copies of Enterprise running as guest operating systems within virtual machines that are hosted on that one physical machine. They all must be used by the same person, however. This exception only applies to the Enterprise edition, however, and the physical PC and all four (or less) virtual machines all must be running the Enterprise edition to stay in compliance with the license.
Jonathan Hassell is an author, consultant and speaker on a variety of IT topics. His published works include RADIUS, Hardening Windows, Using Windows Small Business Server 2003 and Learning Windows Server 2003. His work appears regularly in such periodicals as Windows IT Pro magazine, PC Pro and TechNet Magazine. He also speaks worldwide on topics ranging from networking and security to Windows administration. He is currently an editor at Apress, a publishing company specializing in books for programmers and IT professionals.