Drain pineapple, reserving juice. Place juice in a small saucepan with 1/2 cup water. Bring to a boil; add chilies. Let soak 30 minutes.

Remove stems from softened chilies, then pour chilies and liquid into a blender or food processor. Add 3/4 cup of the pineapple chunks, the garlic, oregano, cumin, salt and vinegar. Puree until smooth.

Slice pork in half horizontally, then chop into small pieces. Place in a shallow glass dish or resealable bag and add chile-pineapple mixture. Cover with plastic wrap or seal bag and marinate in refrigerator at least 1 hour or overnight. Meanwhile, dice remaining pineapple pieces. Refrigerate for later.

Heat oil in a large nonstick skillet over high heat. Scoop pork pieces from the marinade with a slotted spoon and add to pan. Discard marinade. Cook pork for 3 minutes. Add reserved pineapple pieces and cornstarch-water mixture. Cook 1 minute, until pork is cooked through and sauce is thick. Serve mixture in warm corn tortillas, topped with onion, cilantro and sour cream.