1. Combine vinegar, sugar, 1 Tbsp. salt, and 1 cup water in a medium-size glass bowl, stirring until sugar dissolves. Cut potatoes into thin slices, using a mandoline or sharp knife. (Cut them as thin as you can.)
2. Stir potatoes into vinegar mixture; chill 24 hours.
3. Stir together rosemary, pepper, and remaining 1 Tbsp. salt in a small jar. Drain potatoes; gently pat dry with paper towels.
4. Pour oil to depth of 3 inches into a large Dutch oven; heat to 340°. Fry potatoes, in batches, stirring occasionally, 2 to 3 minutes or until golden brown. Drain on paper towels, and immediately sprinkle with rosemary mixture. Serve warm, or cool completely (about 10 minutes).