We prefer a more botanical-tasting gin like Bristow in this punch.
Stir together sugar and 2 cups water in a microwave-safe bowl; microwave at HIGH 5 minutes. Stir to dissolve sugar. Add mint and vanilla bean; cool completely. Discard mint and vanilla bean.
Stir together gin, lemon juice, 2 cups water, and 2 1/2 cups mint-vanilla syrup. Cover and chill 4 hours. Combine gin mixture and sparkling wine in a large punch bowl.