Wash, peel and slice the potatoes. Add them to a large pot and fill with cold water. Bring to a boil over high heat, then reduce the heat to low and simmer until the potatoes are almost fork tender but still slightly firm (about 20-25 minutes). Drain the water and set the cooked potatoes aside to cool.
Meanwhile, whisk the vegan mayonnaise, vegan sour cream (or yogurt), dijon mustard, lemon juice, salt, and white pepper together in the bottom of a large salad bowl until smooth. Add the chopped red onion and sliced scallions to the bowl and toss to coat.
Add the cooked and cooled potatoes to the salad bowl and carefully toss them in the dressing until well coated. Refrigerate for at least 30 minutes before serving.