În a large bowl mix together chicken marinade ( soy sauce, rice vinegar, brown sugar and corn starch) then add chicken and mix well to coat. Refrigerate for 30 minutes up to overnight.
In a small bowl, mix sauce ingredients well and set aside.
Heat a large skillet or wok over high heat add vegetable oil.
Take chicken pieces out, let them drip excess marinade and place chicken in a single layer, do not crowd the pan. Cook for a minute on each side or until chicken gets golden brown.
Remove chicken from pan and set aside. Cook in batches if necessary.
Add mushroom slices and zucchini to the pan and saute for 3 minutes until browned.
Reduce heat to medium and add chicken back to the pan.
Stir sauce well in the bowl then pour over the pan cook for a couple of minutes or until sauce thickens.
Serve immediately over white rice.