Slice cucumber in half lengthwise, trim off ends, and scoop out and discard seeds. Thinly slice cucumber.
In a wide bowl, combine cabbage, carrots, and cucumber.
Mix vinegar, fish sauce, sesame oil, soy sauce, sugar, chili, and garlic. Pour over salad and mix. Serve, or cover and chill up to 4 hours. Sprinkle with peanuts and mix.