Yes, you can. You can install any version of Vista onto a Mac in either Parallels, VMWare Fusion or Boot Camp.
Microsoft has now, however, imposed a legal barrier that you *should* not install the Home Basic version of Vista on virtualisation software such as Fusion and Parallels. Higher end versions of Vista are still allowed though and even Home Basic is allowed on Boot Camp.
That said, you *can* install even Home Basic on virtualisation software, but that would be illegal. Hope that clears the confusion!
anandk, this has already been posted, BTW. And no sane Mac user would consider Vista an upgrade. For most, it is a compromise.