Boil water in a pan and add salt and vinegar to it. Separately take some boiling water in two small steel bowls. Break the eggs and pour the contents into the steel bowls with water. Place the steel bowls in a boiling water pan and allow the eggs to cook for 6 to 7 minutes on a high flame. Cut the buns into two halves horizontally and lightly toast them. Spread tomato sauce on the bottom half of the bun. Top it up with a lettuce leaf. Remove the eggs with the help of a spatula and put them on the lettuce leaves. Sprinkle some salt and crushed black pepper and serve.