For the salad: In a small pan over medium heat, bring the red vinegar to a simmer. Meanwhile, place the onions in a small mixing bowl. When the vinegar comes to a simmer, pour it over the onions (you can add additional water to cover the onions, if needed). Set aside while you prepare the rest of the salad.
Place the cucumbers in a large mixing bowl, season with the salt and toss. Add the tomatoes and gently combine.
For the vinaigrette: Place the basil, oil, shallot and red wine vinegar in a blender and process until smooth. Season to taste with salt and pepper and briefly process again to combine.
Add the dressing to the bowl with the vegetables. Drain the onions (reserve the vinegar for another use or discard) and add them to the cucumbers and tomatoes, tossing everything gently to combine. Garnish the salad with the dill and serve right away.