Set a steamer basket in a large saucepan. Fill with enough water to come just below basket. Bring to a boil; place rutabaga in basket, and reduce heat to medium. Cover and steam 5 minutes. Add potatoes, cover, and steam until vegetables are just tender, about 15 minutes.
Meanwhile, in a large bowl, whisk together vinegar and Dijon; season with salt and pepper. When potatoes are cooked, transfer to bowl with vinegar mixture. Toss to combine; let cool, tossing occasionally.
When potato mixture is cool, mix in oil, celery, and celery leaves; season with salt and pepper.