Marinate the pork in soy sauce, garlic, and pepper for 10 minutes.
Heat vegetable oil in a skillet over medium-high heat.
Stir-fry the marinated pork until browned and cooked through, about 5-7 minutes.
Remove the pork and set aside.
Add onions and bell peppers to the skillet; stir-fry for 3-4 minutes.
Add pineapple chunks and cook for another 2 minutes.
Mix rice vinegar, brown sugar, and cornstarch in a bowl to create the sauce.
Pour the sauce over the vegetables; cook until thickened, about 2 minutes.
Return the pork to the skillet; toss everything together.
Serve hot over rice or noodles.