Vista is really just an update to Windows Server 2003, which is based on Windows 2000 with some features pulled from Windows XP.
OS X has been out since 2001 (1999 if you count the first OS X Server, but then you should count NT for Vista). It's had five updates since then.
It is a fair comparison since both are of relatively equal age.
Oh sorry, I must’ve missed all those ads for a WS2003 update. No, I’m pretty sure this was advertised as a new OS.