In 4-quart saucepan, place potatoes and enough water to cover; heat to boiling over high heat.
Reduce heat to low; cover and simmer 8 to 10 minutes or until potatoes are fork-tender.
Meanwhile, in large bowl, with wire whisk, mix vinegar, oil, mustard, pepper, and 1 teaspoon salt.
Drain potatoes. Add hot potatoes to bowl with vinaigrette; gently stir with rubber spatula to combine. Let stand until cool. In small bowl, with wire whisk, mix mayonnaise, milk, and remaining 1/2 teaspoon salt until smooth.
Add mayonnaise mixture, celery, and red onion to potatoes. Gently stir with rubber spatula to combine.