Set a wok over medium-high heat. Pour the oil around the rim and tilt the pan to coat the sides. Add the onion and stir-fry for 2 to 3 minutes, until translucent. Stir in the garlic. Gradually add the cabbage, continuing to stir-fry for several minutes, until the cabbage is cooked but still slightly crisp.
Remove the wok from the heat and add the vinegar, pepper, and salt; toss thoroughly.