Bring potatoes to a boil and simmer, covered, for 20 to 30 minutes or until just tender. While this is cooking in a glass mixing bowl combine the mayonnaise, yogurt and vinegar. Toss with 1/2 teaspoon salt, freshly ground pepper, red onion, celery, 3/4 cup of bell pepper and 2 of the chopped hard boiled eggs. When potatoes are just tender, drain and pat dry and cube them into 1inch dice. Toss them with ingredients in the bowl and adjust seasoning. Center salad on a bed of lettuce and garnish with remaining chopped egg and bell pepper.