Preheat a grill for 375 degrees F (190 degrees C). Add potatoes. Grill, flipping occasionally, until potatoes are barely just tender when poked with a skewer, about 15 minutes. Let sit until cool enough to handle.
Cook bacon in a skillet over medium heat until grease becomes foamy, about 5 minutes. Remove half the bacon with a slotted spoon; reserve for the topping. Add onion to the rest of the bacon in the pan; cook until translucent, about 5 minutes.
Meanwhile, peel potatoes. Cut into quarters, then into 1/2-inch slices. Place in a bowl.
Stir sugar, salt, black pepper, cayenne, and vinegar in with the bacon. Increase heat to medium-high and bring to a boil. Stir and remove dressing from heat.
Pour dressing over the potatoes; toss to combine. Let sit for 5 minutes. Mix again and add parsley. Top with reserved bacon. Enjoy warm.