Add ham hocks to large pot and add chicken stock, onion, garlic, red pepper flakes, black pepper and enough water to cover ham hocks. Bring to a boil and reduce heat to a simmer, simmer for 1 to 2 hours to create the broth. Take out ham hocks and pull any meat from hocks and replace to pot. Add collard greens and vinegar and simmer from 1 hour to 5 hours depending on the texture you prefer. Salt to taste and serve. I like to serve them in a bowl and add hot or pepper sauce.