1. Bring a large pot of salted water to a boil. Add the diced sweet potatoes and russet potatoes. Cook until tender, about 10-15 minutes. Drain and let cool.
2. In a large mixing bowl, combine the cooled potatoes, celery, and red onion.
3. In a small bowl, whisk together the vegan mayonnaise, Dijon mustard, apple cider vinegar, salt, and black pepper.
4. Pour the dressing over the potato mixture and toss to coat evenly.
5. Garnish with fresh parsley. Serve chilled or at room temperature.