dir /b *.jpg >file.bat
This will give you lines such as:
Vacation2010 001.jpg
Vacation2010 002.jpg
Vacation2010 003.jpg
Edit file.bat in your favorite Windows text-editor, doing the equivalent of:
s/Vacation2010(.+)/rename "&" "December \1"/
That's a regex; many editors support them, but none that come default with Windows (as far as I know). You can also get a command line tool such as sed or perl which can take the exact syntax I have above, after escaping for the command line.
The resulting lines will look like:
rename "Vacation2010 001.jpg" "December 001.jpg"
rename "Vacation2010 002.jpg" "December 002.jpg"
rename "Vacation2010 003.jpg" "December 003.jpg"
You may recognize these lines as rename commands, one per file from the original listing. ;) Run that batch file in cmd.exe.