Place potatoes in a large saucepan and cover with 2 inches of water; bring to a boil. Reduce heat and simmer, uncovered, until potatoes are fork-tender, about 7 to 9 minutes; drain and cool slightly.
Meanwhile, in a large bowl, using a fork, mash yolk from 1 egg with vinegar until smooth. Add yogurt, mayonnaise, relish, salt and pepper; stir until blended.
Add potatoes, celery and onion to bowl; toss to mix and coat. Chop remaining egg white and egg; gently fold into potato salad. Cover and refrigerate until chilled. Yields about 3/4 cup per serving.