Slowly and carefully add vinegar, sugar, Dijon, salt, and pepper to the pot of bacon grease. Place the pot back on the burner, bring the mixture to a simmer, and stir for a couple of minutes. Stir the minced garlic into the mixture and cook for 30 seconds to 1 minute, until the garlic starts to turn a light golden. Remove the pot from the heat and toss in the sliced potatoes, gently mixing until potatoes have absorbed all the liquid. Carefully fold in the cooked bacon pieces and chopped parsley.