In a small non-reactive saucepan, bring vinegar, water and sugar to boil. Remove immediately. Place onion in a bowl. Pour mixture over and set aside for 15 minutes. Place chiles in saucepan and bring to boil in water. Reduce heat and simmer until chiles are soft about 5 minutes. Remove from heat. Reserve cooking water. In food processor or blender puree chiles, mint and half cup cooking water. Lift onions from liquid, add them along with 2 tablespoons of vinegar. Stir to combine. Use immediately or refrigerate for 2 weeks.