In a 2-quart pan, combine sugar, water, lemon zest, cinnamon stick, and vanilla. Bring to a boil over high heat, stirring, and cook until sugar dissolves; continue boiling, uncovered, for 2 minutes. Add pears and bring to a boil again. Cover, reduce heat, and simmer until pears are very tender (about 15 minutes). Remove and discard lemon peel and cinnamon stick.
Transfer pear mixture to a blender or food processor. Add lemon juice and whirl until smooth. Pour into a 9-inch square pan and freeze until edges are frozen but center is still soft.
Spoon mixture into a large bowl and, with an electric mixer, beat until smooth and creamy (or whirl in a food processor). Transfer to a freezer container and place a piece of plastic wrap directly on sherbet. Cover container tightly. Freeze until firm (6 to 8 hours at 0ºF or colder).
To serve, scoop into 6 dessert dishes and garnish with mint.