Add kale to the pan, turn the heat to high and add the liquid. Toss the greens in the oil and stock, then cover and cook fro approx. 5-7 min., until it is soft and wilted, but still quite green. Remove cover and continue to cook, stirring occasionally until all the liquid has evaporated, another 1-2 min. Season to taste with salt and peppers, add vinegar and toss to combine.