In a skillet over medium-high heat, saute the onion in the oil until soft and translucent, about 4 minutes. Add the can of corned beef hash and black beans. Cook until the mixture is heated throughout and slightly crispy, 4 to 6 minutes. Remove from heat.
Fill a large saucepan with 2 to 3 inches of water and bring to a boil. Reduce the heat to medium-low, pour in vinegar, and keep the water at a gentle simmer. Crack an egg into a small bowl then gently slip egg into the simmering water, holding the bowl just above the surface of water. Repeat with the remaining eggs. Cook eggs until the whites are firm and the yolks have thickened but are not hard, 2 1/2 to 3 minutes. Remove the eggs from the water with a slotted spoon, dab on a kitchen towel to remove excess water, and place onto a warm plate.
Cut the arepas in half horizontally. Distribute corned beef hash mixture on the bottom halves of the arepas. Top with slices of tomato and avocado and a poached egg. Close with arepa tops and serve.