The vegetables in this crunchy, satisfying salad can be tossed together with the tangy rice vinegar dressing a week in advance, leaving nothing more to do before serving than to scatter roasted peanuts and fresh herbs over the top.
Bring vinegar, sugar, and 1/2 teaspoon salt to a boil in a small saucepan. Reduce heat to medium and cook until mixture has consistency of syrup, about 5 minutes. Transfer to a medium bowl; let cool completely.
Add cucumber, onion, and chile; toss to combine. Let stand until cucumbers absorb liquid, 5 minutes, then toss again. Salad can be refrigerated, covered, up to 1 week. Top with peanuts and herbs just before serving.