Place the sliced cucumbers, tomato wedges, and onion rings in a large non-reactive glass or ceramic bowl.
In a medium mixing bowl, whisk together the white vinegar, vegetable oil, sugar, water, salt, and black pepper until the sugar and salt are completely dissolved.
Pour the marinade over the prepared vegetables and toss gently to ensure all pieces are thoroughly coated.
Cover the bowl tightly with plastic wrap or a lid.
Refrigerate the salad for a minimum of 2 hours to allow the vegetables to macerate and the flavors to meld.
Toss once more before serving cold.