In a large bowl, combine the sliced cucumbers, red onion, and chopped tomatoes.
In a separate small bowl, whisk together the white vinegar, olive oil, sugar, salt, black pepper, and dried oregano until well combined.
Pour the dressing over the vegetable mixture and toss gently to coat all the ingredients evenly.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavors to meld.
Before serving, give the salad a gentle toss and sprinkle with fresh parsley if desired.