1. Whisk together the vinegar, sugar, coriander, red pepper flakes, and salt in a salad bowl until the sugar is dissolved. Stir in the cucumbers, onion, tomatoes, cilantro, and chopped mint, and toss to coat with dressing. Cover and refrigerate for 1 hour to blend the flavors.
2. Before serving, toss again with chopped peanuts, and garnish with sprigs of fresh mint.