As MisterMe said.
The only legal way to run both Mac OS X and Vista on the same computer is when the computer in question happens to be a Mac and running Mac OS X, and then use Boot Camp and/or VMware Fusion or Parallels to run Vista. In case of Boot Camp you can boot to either, or with the other two softwares (which do support Boot Camp partition too) you could virtualize the OS and run the Windows OS while you are booted in Mac OS X.
The legal note being as Mac OS X can only be installed on Apple-branded computer as defined by its user license.