1. Bring 1/2 cup sugar and 1/2 cup water to a boil. Reduce heat; simmer, stirring until sugar dissolves, about 3 minutes. Transfer to a bowl; chill for 1 hour.
2. Place 2 oz. vodka in a shallow dish. Place remaining 1/2 cup sugar in a separate shallow dish. Dip glass rims in vodka; coat in sugar.
3. Pour juice into a bowl. Peel, core and cut pears into small pieces; toss in juice. Place fruit, juice, 2 oz. vodka and cooled syrup in blender. Blend until smooth.
4. Spoon 2 to 3 Tbsp. pear puree into 10 glasses; top with prosecco. Serve right away.