Quarter cabbage, and discard core. Shred cabbage as finely as possible with a sharp knife. Transfer to a large bowl.
In a small bowl, stir together vinegar and sugar until the sugar has dissolved. Season with salt and pepper. Whisking constantly, add the oil in a slow, steady stream. Drizzle vinaigrette over the cabbage, and toss to combine. Let cabbage stand, loosely covered at room temperature, one to two hours, tossing occasionally. Just before serving, sprinkle cabbage with poppy seeds.