In a large nonreactive saucepan combine vinegar, water, sugar, and salt. In a large heatproof bowl combine chiles and red onion. Using a mandoline, slice potatoes about 1/16-inch thick; add to vinegar mixture. Bring to boiling; reduce heat. Simmer, uncovered, 2 minutes. Pour vinegar mixture over onion mixture. Let stand 1 hour before serving, or cover and refrigerate up to 1 week.