Prepare all your vegetable first. Cut small chunks from the sides of the cucumber so that you avoid the middle seedy part. Discard the seeds. Finely slice the pale part of the spring onion and then slice the green parts into batons. Cut up all the remaining vegetables too. Whisk together fish sauce, white vinegar, sugar and cornflour. Set aside for later. Heat vegetable oil in a wok over high heat. Add garlic and onion and stir-fry for a few seconds. Then add the chicken and stir-fry for 3-4 minutes or until almost cooked. Add the capsicum and stir-fry for half a minute. Then add the tomato, cucumber and pineapple and stir-fry for another minute. Pour over the fish sauce mixture and stir-fry until thick and glossy. Remove from heat and toss through green part of the spring onions.