Place cabbage and green onions in a large bowl. In a blender, combine vinegar, soy sauce, ginger, garlic, chili flakes, and sugar, blending until smooth. With the motor running, slowly add the oil. Season with salt and pepper, to taste. Pour the dressing over the cabbage and gently toss. Cover and refrigerate for 2 to 3 days before serving to let the cabbage wilt and the flavors meld.