Place potatoes in a large saucepan covered with 1-inch (2 ½ cm) of water. Season water liberally with salt and bring to a boil over high heat. Cook until tender, about 15 minutes. Strain and set aside to cool slightly.
Add vinegar, sugar and ½ tsp (3g) salt to a small saucepan over medium heat. Bring to a boil and simmer until sugar and salt are dissolved, about 2 to 3 minutes. Place shallot into a small heatproof jar or container. Add vinegar mixture and let stand 10 minutes.
Whisk together Dijon, lemon zest and juice, 1 tablespoon (15ml) pickling vinegar and oregano in a large bowl. While whisking, slowly stream in oil. Season with salt and pepper. Add warm potatoes to dressing with capers and parsley. Strain shallots and add to salad. Season with salt and pepper to taste.