Slice the zucchini into 1/4-inch rounds, then slice celery stalks on a bias. Finely dice the red onion and green bell pepper.

In a mixing bowl, combine sugar, rice vinegar, wine vinegar, vegetable oil, and water. Crush the dried basil and whisk in salt, black pepper, and optional red pepper flakes until sugar dissolves.

Add the prepared vegetables to a large bowl. Pour the dressing over the top and toss gently until well-coated.

Cover and refrigerate for 90 minutes to allow flavors to meld.

Before serving, drain excess liquid, stir, and serve cold.