Heat a sauté pan over medium heat; add bacon and cook for 2 minutes.
Add the onion and kale. Sauté for 3 to 5 minutes, until the onion begins to wilt and the kale is bright green.
Add the water and reduce heat to low. Cook, stirring occasionally, until the kale is tender (about 10 minutes). Add a little more water if needed. Remove from heat and stir in the vinegar, salt and pepper.