Simmer red, white and blue potatoes, skins on, in salted water until tender but not mushy. Let cool until easy enough to handle, but still warm. Cut into 1-inch pieces.
Combine mayonnaise, ranch dressing, Champagne vinegar, dill weed and optional sugar. Pour over warm potatoes and toss to coat.
Season with salt and pepper to taste. Refrigerate 2 hours or more to let flavors meld.
Let come to room temperature to serve. Garnish with grape tomatoes. Recipe may easily be doubled.
Notes: Waxy potatoes such as red rose, white rose, blue and gold potatoes are best for potato salad because they are not grainy or mealy and hold together well. You may substitute chopped chives for the green onions.