In a medium-sized mixing bowl, thoroughly whisk together the sour cream, red wine vinegar, fresh dill, granulated sugar if using, and garlic powder until the mixture is smooth and well blended. Adjust the seasoning with salt and pepper to your preference. Incorporate the sliced cucumbers and red onion into the prepared dressing, folding gently to ensure all slices are evenly coated. Serve promptly at room temperature, or for a cooler option, chill in the refrigerator for up to one hour before presenting.