Add vinegar, pepper flakes, and sugar to a small sauce pan. Bring vinegar to a boil and stir sugar until dissolved (3 minutes). Remove from heat and let cool completely.
Combine tomatoes, onion, and cucumbers in a bowl. Sprinkle with salt and let sit at room temperature for an hour.
Once the vinegar is cooled, combine with vegetables and let sit in the refrigerator for at least 2 hours before serving.
Recipe by a Cook at Heart at https://acookatheart.com/2014/07/cucumber-tomato-salad/