In a large saucepan, bring the pears, water, lemon juice, sugars, and bay leaf to a boil over moderate heat. Lower heat and simmer, uncovered, for 25 minutes or until the pears are very tender. Let cool then remove and discard the bay leaf and stir in the vanilla.
Transfer mixture to a food processor (or blender) and whirl for 1 minute or until pureed. Pour into an 8x8x2 pan, cover with plastic wrap and freeze for 2 hours until the center is almost frozen. Remove the mixture from the freezer and beat until smooth. Return to the freezer for 45 minutes and beat again until smooth. Freeze 2-3 hours more or until firm.