This cocktail is traditionally made with a Spanish lemon-lime soda that is less sweet than most available in the U.S. We found that combining fresh lemon juice and seltzer water, then upping the vermouth, not only made up for the loss, but also kept our cocktail tasting lighter and fresher. We loved the flavor of Spanish Lustau vermouth here, but any sweet vermouth works. Just make sure it's something you enjoy on it's own, as its flavor dominates the cocktail.