Cook 4 slices chopped bacon in a large pot until crisp; drain on paper towels. Add 1 diced onion and 2 teaspoons Cajun seasoning to the pot with the drippings; cook, stirring, 5 minutes. Stir in 2 bunches chopped collard greens in batches. Add 4 cups each chicken broth and water and bring to a boil. Reduce the heat and simmer until the greens are tender, about 2 hours. Stir in the bacon. Add cider vinegar, hot sauce, salt and pepper to taste.