Prepare the Vegetables
Wash and slice cucumbers thinly. Slice red onion and halve the cherry tomatoes if using. Chop fresh parsley or dill.
Mix the Dressing
In a small bowl or jar, whisk together vinegar, olive oil, sugar, salt, pepper, and lemon juice until emulsified.
Combine the Salad
In a large bowl, add cucumbers, onion, herbs, and tomatoes. Pour the dressing over and toss gently to coat.
Chill Before Serving
Cover the bowl and refrigerate for 20 minutes to allow flavors to meld. Stir once before serving.