Slice bagels in half horizontally and toast until golden. Meanwhile, place a 12-inch skillet over medium-high heat and add enough water to come about 2 inches up the side of the skillet. Add the vinegar and a teaspoon of salt and bring to a simmer. Crack each egg gently into a ramekin or similar small dish then transfer the egg very carefully into the simmering water in the skillet. Repeat until all four eggs are in. Turn off heat, cover the pan and set a timer for 5 minutes. Resist the temptation to peek and poke! Meanwhile, slice the avocado and lay several slices onto the lower half of each bagel. Gently remove each egg with a slotted spoon and allow excess water to drip before transferring each egg to the bottom half of each bagel. Sprinkle the poached egg with salt, pepper, and a pinch of chile flakes. Gently place the top half of the bagel onto the bottom half and serve.