Windows Vista doesn't =Windows 7 Professional
Radenight and Drew gave you links to charts that show which 7 OS is considered equal to which Vista OS, but didn't point out that you made an incorrect statement in your original post.
Vista Home Premium=Windows 7 Home Premium
Vista Business=Windows 7 Professional
If one is moving from Vista Home Premium to Windows 7 Professional, for instance, they must pay full version price to move to the higher version. The upgrade prices only apply to "same level" upgrades Vista Home Premium to 7 Home Premium, Vista Business to 7 Professional, etc.
I made a typo in the title here. I meant to say that Windows Vista Home Premium does not = Windows 7 Professional