Marinate the pork pieces in soy sauce for 10 minutes.
Heat vegetable oil in a large pan over medium-high heat.
Add the marinated pork and cook until browned on all sides.
Remove the pork and set aside, keeping it warm.
In the same pan, add onions and garlic; sauté until fragrant.
Add bell peppers and cook until tender-crisp.
Stir in pineapple chunks and cook for another 2 minutes.
In a bowl, mix vinegar, ketchup, brown sugar, and cornstarch.
Pour the mixture into the pan, stirring to combine.
Add the cooked pork back to the pan, stirring until well coated and sauce thickens.