Preheat a grill to medium. Put the red potatoes in a large saucepan and cover with cold water by 2 inches. Bring the water to a boil over high heat. Once the water begins to boil, after about 10 minutes, add the sweet potatoes. Return to a boil and cook 10 more minutes. Drain the potatoes and set aside to cool.
In a medium bowl, whisk the mayonnaise, mustard, onion powder, garlic powder, cayenne, and salt and black pepper to taste. When the potatoes are cool to the touch, gently toss them in the mayonnaise mixture to coat.
Make the vinaigrette: Cook the bacon in a small skillet over medium heat until crisp, then set aside to cool in the skillet. Once cool, pour the bacon and drippings into a small bowl along with the olive oil. In a large bowl, whisk the vinegar and mustard. Slowly drizzle in the olive oil-bacon mixture, whisking to emulsify. Add the scallions, parsley, and salt and black pepper to taste and whisk to blend.
Using tongs, place the potato pieces on the hot grill and cook on all sides long enough to cook through and make grill marks, 1 to 2 minutes per side. Handle the potatoes gently so they don't fall apart. Remove the potatoes from the grill and place in the bowl with the vinaigrette. Gently toss to coat completely and serve warm or cold.
Photograph by Kat Teutsch
Recipe courtesy Melissa d'Arabian for Food Network Magazine