You cannot change an OS drive letter as it will ruin it in every case.
If Windows 7 is not C when booted it is because the installer was incorrectly run from Vista which blocked out the availability of the letter so that Windows 7 couldn't assume C whenever booted, as it does when it's installer is correctly booted.
If you need to remove Vista then we need to see the requested screenshot to advise you the exact steps as there are unknowns that need to be seen first. Use the Snipping Tool in Start menu, maximize Disk Mgmt, capture a rectangular snip of the drive map with listings, post back using Paper Clilp in reply box. Screen Shots