In a small bowl, whisk together the white wine vinegar, whole grain mustard, sugar, salt, and pepper. Set aside.
Place the potatoes in a large pot and fill with water about an inch above the potatoes. Bring the water to a boil; reduce the heat to medium, maintaining a gentle simmer. Simmer for 8 to 12 minutes, or until the potatoes are soft and fork-tender.
Drain the potatoes in a colander, and rinse with cold water. Set aside.
Return the pot to the stove over medium heat. Add the chopped bacon and cook until browned, about 4 to 5 minutes. Remove the bacon, and set aside on a paper-towel lined plate.
Discard excess bacon grease until only about 2 tablespoons remain in the pot. Add onion and garlic, and cook until slightly softened, about 2 minutes.
Turn the heat to medium-low, and pour in the prepared vinaigrette dressing. Stirring constantly, cook until the dressing is warmed through and the sugar is dissolved, about 1 minute.
Add in the potatoes and bacon. Gently fold until the ingredients are evenly coated in the dressing.
Remove from the heat and allow to sit for 10 minutes before serving, to allow the dressing to settle into the potatoes.
Taste for salt and pepper, and adjust accordingly.
Garnish with parsley and serve.