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).
In addition to the total time, allow 24 hours for brining.
Make Ahead: Fry chips up to 1 day ahead, let cool, and store in an airtight container. Re-crisp and warm the chips on a jelly-roll pan in a 300° oven for 20 to 25 minutes.