Faulting an AV-less Vista for not stopping viruses is a bit like faulting a door without a lock for opening when the handle is twisted.
That is not right. If I build a house with a front door and a reasonable good lock, there iss e difference to a house in the woods with no lock at all. Software architecture can sure make a difference if it is easy for malware to take unwanted advantage. You don't have to have some AV Software on the system installed, if the OS would only allow signed code from trusted sources to execute.
But then again you have other problems.
;)
My 2cents: M$ could have made Vista more secure out-of-the-box without AV-Software.