Combine the Simple Syrup, gin, vermouth and lemon juice in a shaker with ice. Shake and strain into 4 champagne flutes. Top each glass with some champagne and serve with a lemon twist.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil. Cook, stirring, until the sugar dissolves. Remove from the heat and let cool completely. Keep covered in the refrigerator up to 1 month.