Combine pineapple juice, sugar, lemon juice, cornstarch and soy sauce in a bowl.
In a large saucepan or Wok heat vegetable oil over medium heat.
Add onion, broccoli, carrots and celery. Stir and cook for about two minutes.
Add the zucchini and peppers. Stir, cover the pan and cook for one minute.
Add sauce, bring to a boil and cook for one minute more, covered.
Serve vegetables while hot.