You don't say what the Vista version is. The only in-place upgrade path to Windows 7 Professional is Vista Business. There may be a workaround however. Worst case is you'll have to Clean Install Windows 7
which is a superior install anyway.
Extract the ISO from your Windows 7 DVD using ImgBurn
or download the latest from Digital River official Windows 7 ISO page. Run the eicfg removal tool
to unlock all versions then burn another DVD using ImgBurn at 4x speed.
Back up your files and a Vista backup image if you want a path back.
Boot into the Vista OS, put the Windows 7 DVD in drive, choose Home Premium, run the in-place Upgrade to Windows 7 Home Premium.
From the Windows 7 desktop, type regedit in Start Search box, rightclick result to Run As Administrator, browse to the two Registry keys shown to change them to exactly as shown in screenshot below. This tricks the installer into doing an in-place Upgrade to Professional since it thinks it's doing a same-version Repair Install
click to enlarge
Now run the Pro installer from the Windows 7 Home Premium desktop to do the in place Upgrade to Professional.
One report I saw said the user was able to install the Pro Key in an Anytime Upgrade which simply unlocks the extra features, even though Home Premium isn't activated. You can try that first by typing Anytime in Start Search box.