Place bacon in a large heavy-bottomed pot over medium-high heat. Cook until browned and most of the grease has been released. Stir in onion and cook until tender and glistening, about 4 minutes.
Place collards into the pot and stir to coat with the bacon drippings. Pour in the chicken stock. Season with cayenne pepper. Reduce heat to low and cook for 1 hour and 15 minutes.
Stir red wine vinegar into the pot. Continue cooking 15 minutes, until liquid is reduced by about 1/2.
