Fill a medium pot with about 3 inches of water and bring to a gentle simmer. Add the vinegar.
While the water heats, toast your bread slices until golden brown.
In a bowl, mash the avocado with lemon juice, salt, and pepper until you reach your desired consistency. I prefer leaving some chunks for texture.
Create a gentle whirlpool in the simmering water with a spoon. Crack each egg into a small bowl first, then carefully slide it into the water.
Poach the eggs for 3 minutes for runny yolks, or 4 minutes for firmer yolks.
While eggs are poaching, spread the mashed avocado mixture on your toast slices.
Remove eggs with a slotted spoon, letting excess water drain off.
Place one egg on each piece of toast, season with salt, pepper, and red pepper flakes if desired.