Prepare the Cucumbers: Rinse and dry the cucumbers: Halve them lengthwise, then cut them into slices about 1/4 inch thick. The slices should be thick enough to remain crisp after marinating.

Cut the Tomatoes and Onion: Cut the tomatoes into 1-inch wedges, removing any tough stem sections. Slice the red onion as thinly as possible so it softens slightly in the dressing.

Make the Vinegar Dressing: In a small bowl or measuring cup, whisk together the red wine vinegar, olive oil, sugar, salt, and black pepper. Continue whisking until the sugar and salt are mostly dissolved.

Combine the Vegetables: Place the cucumbers, tomatoes, red onion, and chopped parsley in a large nonreactive bowl. Gently toss to distribute the onion and herbs without crushing the tomatoes.

Dress the Salad: Pour the vinegar dressing evenly over the vegetables. Fold gently with a large spoon until every piece is lightly coated.

Chill the Salad: Cover the bowl and refrigerate for 30 minutes. Stir once after about 15 minutes so the vegetables marinate evenly.

Check the Texture and Seasoning: Stir the salad again before serving. The cucumbers should remain crisp, the onion should taste less sharp, and a small amount of vegetable juice will have collected in the dressing. Taste and add a small pinch of salt only if needed.

Serve: Transfer the salad to a serving bowl with a slotted spoon for a less juicy presentation, or include some of the dressing for a more marinated texture. Serve chilled.