Whisk together the vinegar, lime juice, 1 C yogurt, 1/2 C oil until smooth. Puree the dressing, bread, cucumber, tomatillos, scallions, jalapenos, and garlic in a blender until smooth. Season to taste, cover and chill at least 4 hours.
Whisk the remaining 1/2 C yogurt, thinning with water until the consistency of heavy cream. Season to taste.
Plate the soup, garnish with the thinned yogurt and more oil, and serve cold.