The sauce can stand at room temperature up to 8 hours until serving. Lime juice dulls and can make the sauce slightly bitter if left longer; combine water, sugar, and fish sauce to make a base, then refrigerate, covered, up to 2 weeks. (Prep a double batch if you use it a lot.) To finish, add lime juice, vinegar (if using), and any desired add-ins.
VARIATIONS: For a vegetarian nuoc cham: Stir together 3 tablespoons light brown sugar (or 4 to 5 tablespoons pure maple syrup), 3 tablespoons fresh lime juice, and a rounded 1/2 teasopon finely ground sea salt in a small bowl until sugar dissolves. Taste and add additional sugar or up to 1 teaspoon rice vinegar to round out the lime juice. Stir in 2/3 cup lukewarm water and 1 1/2 teaspoons soy sauce. Finish with any add-ins before serving.