Take a wide, shallow saucepan about 3 inches deep and bring the water to a boil with the vinegar. Meanwhile, crack the egg into a ladle. As soon as the water starts boiling, create a whirlpool using a whisk by stirring counterclockwise quickly with your right hand, while sliding the egg into the water with your left hand. It will thicken in a few minutes. Gently place it on the toasted bread using a slotted spoon, making sure to drain the water well.