I think it's possible to do a FreeVista OS. Just llok at all theses emulators. They all emulates old consoles, computers, etc.
Full teams of engineers worked to create the Genesis, the Nintendo entertainment system, etc, and 2, 3 persons can make an emulator. Oh, you can say It's easier since the architecture is conceived, but...
Windows Vista seems like something Microsoft didn't know what to do with it. It's not like Windows 95, for example, or Windows XP. They didn't think enough in making it, and I think in the future, in a couple of years when we will be more intelligent, making a Windows Vista clone will be quite easy, if doing something like that will be wanted.
But I think Vista is on the domain of the technological leadership the same as Windows ME on the technology period.
It may be pretty, the search thing is good, but it gives the impression, to me, someone who didn't even tried the OS, that Microsoft wanted to do an OS, but didn't know what they were doing en route.
So I have a question for you, are you trying to emulate Microsoft completely on this one?
Personnaly, what I would do, is a Window Manager for the X Window System or what it's called, reproducing the Windows Vista theme, even if it's "not legal".