Chill the sparkling wine, gin, and vodka in the refrigerator for at least 1 to 2 hours. Meanwhile, squeeze fresh oranges until you have about 1¼ cups of juice.
In a large pitcher (at least 6 cups), combine the orange juice, gin, and vodka. Add sugar, if using, to taste, and stir until completely dissolved.
Just before serving, open the chilled sparkling wine and gently pour it into the pitcher. Stir very gently to keep the bubbles intact.
Fill glasses or tumblers with ice, pour the cocktail over the ice, garnish each glass with an orange slice, and serve immediately.