Add vinegar, soy sauce, Chinese hot oil and sugar. Stir and boil for a few seconds to dissolve sugar and remove from heat.
Cook capellini per packaged instructions. Drain and shake the noodles to remove water. Immediately toss with seasoned oils. Toss until noodles are all well coated and not sticking together. Cover and refrigerate until ready to serve. Noodles are even better if made a day in advance, but go ahead and eat them right away.
Before serving add chopped cilantro and green onions and any optional ingredients. Toss again. Serve cold or room temperature.