Wash the cherry tomatoes, red onion, and cucumber. Slice the red onion and cucumber thinly. In a small saucepan, combine the white vinegar, water, sugar, salt, oregano, and red pepper flakes. Bring to a boil over medium heat, stirring until the sugar and salt are dissolved. Place the cherry tomatoes, red onion, and cucumber in a jar. Pour the hot pickling liquid over the vegetables, ensuring they are fully covered. Add the fresh dill to the jar. Seal tightly and refrigerate for at least 2 hours or up to 2 weeks. The pickled vegetables will develop more flavor the longer they chill.