In a large pot, cover potatoes with water and season generously with salt. Bring water to a boil and cook until potatoes are easily pierced with a knife, 15 to 20 minutes. Drain and let cool slightly before transferring to a large serving bowl. In a large skillet over medium heat, cook bacon until crispy, about 8 minutes. Reserve about 2 Tbsp. fat in pan, and transfer bacon to a paper towel-lined plate to drain. Add red onion and cook over medium heat until starting to soften, about 3 minutes. Whisk in in apple cider vinegar, water, olive oil, Dijon mustard, and sugar. Bring mixture to a simmer, then remove from heat; season with salt and pepper. Stir in green onions and gradually pour mixture over potatoes. Crumble bacon over potatoes. Toss to combine and serve warm.