Prepare the Vegetables:
Wash and slice the cucumbers into thin rounds. Peel and thinly slice the red onion. Chop the tomatoes into bite-sized pieces. Place all of the vegetables in a large mixing bowl.
Make the Marinade:
In a separate small bowl, whisk together the white vinegar, olive oil, sugar, salt, black pepper, oregano, and garlic powder until the sugar dissolves and the dressing is well combined.
Toss and Marinate:
Pour the marinade over the cucumber, onion, and tomato mixture. Toss gently to coat all the vegetables evenly in the dressing.
Chill:
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld. For best results, let it marinate for 1–2 hours.
Serve:
Garnish with fresh parsley or dill, if desired. Serve chilled as a refreshing side dish or light snack.