Boil potatoes in salted water for 12 minutes, until fork tender. Drain and return potatoes to hot pot to dry them. Sprinkle potatoes with vinegar. Transfer potatoes to a bowl and let them cool.
When potatoes have cooled, add pimento and peas to the bowl. Using a hand held grater, grate the small white onion over the potatoes. The onion juice is what makes this salad so good. Add 1/2 cup of mayonnaise dressing to the bowl, or to your liking, to bind.
Season potato salad with salt and pepper, to taste. Transfer to plastic serving wear and pack up for your picnic. Keep this salad in your cooler with beverages or chill until ready to travel.