You can't legally VM Mac OS. It just doesn't have compatible licensing.
You can if it's 10.7 or newer and your host is a Macintosh itself.
Prior to 10.7, you had to run the server flavor of the OS and be on Macintosh hardware. Which is why I own a copy of OS X Server 10.6 -- lets me run it in a VM.
But after 10.7 came out, Server changed to an app you run on top of the regular OS instead of a distinct version of the OS, and they updated the licensing at that time.
If you're confused about what you are and aren't allowed to do: VMWare keeps track, and if you have the latest build of VMWare Fusion, it'll only let you install versions you're allowed to run.