Not so long ago, the otherwise savvy cocktail connoisseur put less than a thimbleful of thought into vermouth brands. After all, the choices were few and the standard proportions—such as in a desert-dry Martini—were sparing. Even the choice of garnish may have posed a greater concern. But an explosion in the vermouth category has made it necessary for mixologists to take a closer look at the spirit. Not only are new examples from old stalwarts and previously unfamiliar brands emigrating from the traditional European production regions, but American makers are showcasing versions with striking variety in flavor. Wine Spectator's Jack Bettridge explains.