In a Dutch oven or large pot, combine the potatoes, water, salt, and vinegar and bring to a boil over high heat. Reduce heat to medium and cook at a strong simmer until potatoes are just tender, about 10-15 minutes.

Meanwhile, in a large bowl, whisk together the lemon zest and juice, salt, pepper. Slowly whisk in the oil until combined; set aside.

Drain potatoes, then transfer them to a rimmed baking sheet. Drizzle 2 tablespoons of the dressing over the hot potatoes and toss gently to coat. Let potatoes cool to room temperature, about 30 minutes, stirring once halfway through cooling.

Whisk dressing to recombine and stir in onion, tarragon, parsley, green onions, and capers. Add cooled potatoes to dressing and stir gently to combine. Season with salt and pepper to taste. Serve.