Place 4 large martini glasses in the freezer. Combine the sugar and lemon juice in a small bowl; whisk until the sugar dissolves. (The lemon syrup can be made 3 days ahead. Cover and chill.)
Combine 1/4 cup lemon syrup and the vodka in a martini shaker. Add ice cubes. Cover and shake 8 to 10 times, then strain the mixture equally into 4 very cold martini glasses (about 3 tablespoons each). Add 1/4 cup chilled sparkling wine to each glass. Garnish each martini with a lemon slice if desired, and serve very cold.