Combine 1/3 cup water, the vinegar, sugar and a big pinch of salt in a small microwave-safe bowl or liquid measuring cup. Add the dried cranberries. Microwave until the liquid starts to simmer, 1 1/2 to 2 minutes. Let the cranberries sit in the liquid for 30 minutes to pickle slightly, then drain.
Meanwhile, preheat the oven to 375 degrees F. Combine the butter and sage in a small microwave-safe bowl and microwave until melted, about 1 minute. Toss with the bread pieces on a baking sheet; season with salt and pepper and spread in a single layer. Bake, tossing halfway through, until crisp, 15 to 20 minutes.
Make the dressing: Combine the buttermilk, sour cream, mayonnaise, mustard, garlic, 1/4 cup dill, the parsley, 1 scallion, 3/4 teaspoon salt and a few grinds of pepper in a food processor. Pulse to make a smooth dressing.
Gently toss the lettuces and the remaining 1/4 cup dill and 3 sliced scallions with the dressing in a large bowl; season with salt and pepper. Sprinkle with the cranberries and croutons and gently toss.