1. Soak bread in water and vinegar, stirring occasionally until softened, about 7 minutes.
2. Preheat broiler. Cut sides and bottom from pepper, remove stem and seeds, and arrange in one layer on a baking sheet. Brush pepper with some oil and broil 5 to 6 inches from heat, turning until blistered and blackened in spots and tender, about 5 minutes total. Let cool.
3. Transfer half of peppers, tomatoes, bread mixture, juice, garlic, and 1/2 cup cucumber to a blender. Blend mixture until smooth and transfer to a bowl. Blend remaining peppers, tomatoes, bread mixture, juice, garlic, and 1/2 cup cucumber until smooth. Add to bowl and whisk in 3 tablespoons oil. Season with salt and pepper to taste.
4. Chill gazpacho, covered, for at least 1 hour. Pour soup into little cups and, just before serving, garnish with cilantro and chopped onions, and sprinkle with remaining 1/4 cup cucumber. Drizzle with remaining oil.