Using a vegetable peeler, remove zest from tangelos in long strips. Cut the strips lengthwise into very thin pieces. Cut the tangelos in half and squeeze enough juice from them to get 1/2 cup.
Heat a large wok or skillet over medium-high heat. Swirl in 2 teaspoons oil, then add pork and cook, stirring, until just cooked, 2 to 3 minutes. Transfer to a plate. dd the remaining 1 teaspoon oil to the pan along with shallots, garlic, ginger, crushed red pepper and the zest. Cook, stirring, for 1 minute. Add bell peppers and celery and cook, stirring constantly, until crisp-tender, about 2 minutes. Stir in the tangelo juice and soy sauce; bring to a simmer. Cook for 1 minute.
Whisk vinegar and cornstarch in a small bowl, then pour it into the pan along with the pork and its juices. Cook, stirring often, until thickened and bubbling and the pork is heated through, about 1 minute.