Separate the shallot into rings. Place in small bowl and cover with 3 tablespoons of red wine vinegar. Allow to sit at room temperature for at least 30 minutes and up to 24 hours. (Cover with wrap if leaving for extended time.)
When ready to make the salad, combine remaining 3 tablespoons of red wine vinegar with olive oil. Season with salt and pepper and whisk to blend. Set aside.
Slice tomatoes into 1/4-inch discs and place decoratively on a platter.
Slice cucumber in half lengthwise, use a spoon to scoop out seeds, and then slice halves into 1/8-inch crescents. Arrange among the tomato slices.
Lift shallots from red wine vinegar with fork and scatter on top of tomatoes and cucumbers. Discard soaking vinegar.
Sprinkle half of fresh herbs on top of tomato and cucumber mixture, distributing evenly.
Add remaining chopped herbs to vinaigrette and whisk to combine.
Drizzle vinaigrette over tomato and cucumber mixture. Season with salt and pepper to taste.