Put 2 pounds chopped red-skinned potatoes in a pot, cover with water by 2 inches and add salt. Bring to a boil and simmer until tender. Transfer the potatoes to a bowl using a slotted spoon; sprinkle with salt and 2 tablespoons cider vinegar. Boil 10 ounces frozen peas in the same pot until just tender. Drain, rinse and add to the potatoes. Whisk 3/4 cup each mayonnaise and sour cream, and 1/2 cup each chopped chives and dill. Toss with the potatoes and peas; season with salt and pepper.