From what I recall, since the Mac uses an EFI Bios, you can't just format the system and pop in a Vista cd and be on your way. You still need Bootcamp to emulate a PC Bios on the system so that you can run Vista/XP in bootcamp. And you can only install bootcamp if you have an Intel Mac running Leopard.
I suppose once you have bootcamp set up, you could delete the OSX Partition, but I'm not sure how well it would work, if at all. The software for the PC Bios emulation may very well be contained on the OSX Partition, but I am not sure.