In a large bowl, combine the sliced cucumbers, red onion, and chopped tomatoes.
In a separate bowl, whisk together the white vinegar, olive oil, sugar, salt, black pepper, and dried oregano until well combined.
Pour the dressing over the cucumber, onion, and tomato mixture. Toss gently to coat all the vegetables 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.