Whisk vinegar, water, sugar, olive oil, salt, pepper, dill, and garlic in a bowl until the sugar dissolves.
Slice the cucumbers and red onion thinly. Cut the tomatoes into wedges.
Combine cucumbers, tomatoes, and onion in a large mixing bowl.
Pour the marinade over the vegetables. Toss to coat well.
Cover and chill 20–30 minutes, tossing once. Taste, adjust seasoning, and garnish with parsley before serving.