Chop potatoes and eggplants. Dip the chopped eggplants into vinegar and cornstarch. Set aside.
Heat up a large pan to high heat. Add oil. Deep-fry the chopped potatoes. Then add the chopped eggplants. Deep-fry again. Drain the veggies. Set aside. In the same pan, add chopped bell peppers into colander. Fry for a while. Drain it and set aside.
In the same pan, turn down to medium heat. Add minced scallion and minced garlic, whisking in oyster sauce, soy sauce and stir-fry for a while. Pour in some water and cornstarch mixture. Seasoning with salt and pepper.
Put the fried veggies back into the pan. Stir-fry for a while and cook well.