Make the dressing: Stir vinegar, lime juice, and sugar together in a small bowl until sugar dissolves.
Make the salad: Hold cucumber upright and use a peeler to remove strips of peel lengthwise, making a pattern of peeled and unpeeled strips all the way around for a decorative effect. Cut cucumber in half lengthwise, then thinly slice each half.
Place cucumber into a salad bowl with tomatoes and red onion; mix to combine. Pour dressing over salad and mix until well combined. Cover and refrigerate for at least 30 minutes.
Just before serving, stir cilantro into salad and sprinkle chopped peanuts over top.