Fill a medium saucepan with 3 inches of water and bring to a gentle simmer. Add the white vinegar.
Crack one egg into a small bowl. Create a gentle whirlpool in the water and carefully slide the egg into the center. Poach for 3-4 minutes until whites are set but yolk is runny. Remove with a slotted spoon and drain on paper towels. Repeat with second egg.
While eggs poach, toast the bread slices until golden and crisp.
Cut the avocado in half, remove the pit, and scoop the flesh into a bowl. Mash with lemon juice, salt, pepper, and olive oil until creamy.
Spread the mashed avocado evenly over the toasted bread.
Place a poached egg on top of each avocado toast.
Sprinkle with additional salt, pepper, red pepper flakes if using, and garnish with chopped chives or parsley.
Serve immediately for best taste and texture.