Preheat the oven to 350 degrees F.
Place the pancetta slices on a rimmed baking sheet and bake until crispy, about 13 minutes. Remove from the oven, let cool, then crumble.
Whisk together the vinegar, oil and some salt and pepper in a small bowl. Taste and adjust the seasoning as needed.
Combine the tomatoes, cucumbers and onions in a large bowl and toss to combine. Pour the vinaigrette over; toss to coat. Season with salt and pepper.
Let the salad sit at room temperature for about 30 minutes, tossing occasionally. Just before serving, scatter with the crumbled pancetta.