Peel and discard the outer, fibrous skin layers of banana heart until you reach the lighter, softer core. Trim off stem and discard. Cut banana core into half and slice thinly. In a bowl of cold salted water, place shredded banana and soak for about 15 to 20 minutes. Using hands, squeeze to release bitter sap. Rinse with cold water and drain well, discarding liquid.
In pan over medium heat, heat oil. Add onions and garlic and cook until limp and aromatic. Add pork and cook, breaking apart with back of spoon, until lightly browned. Add chili peppers. Add banana heart. Season with salt to taste and generously with pepper. Continue to cook, stirring occasionally, for about 5 minutes or until softened and releases liquid. Add water and bring to a boil. Lower heat, cover and cook for about 10 to 15 minutes or until tender.
Add oyster sauce and stir to combine. Continue to cook until liquid is reduced and vegetable and pork are cooked through. Remove from heat and drain well in a colander.
Separate wrappers into individual sheets. On a flat working surface, lay wrapper like a diamond. Arrange about 1 heaping tablespoon of vegetable mixture in the middle of diamond. Fold bottom pointed end of wrapper over filling. Fold side ends of the sheet inward and roll into a log. Wet the pointed edge of the wrapper with water to completely seal. Repeat with the remaining mixture.
In a skillet over medium heat, heat about 1 inch deep of oil. Add lumpia seam side down and fry, turning once or twice, for about 3 to 5 minutes on each side or until golden brown and crisp. Remove from pan and drain on wire rack set over a baking sheet. Serve hot with a spiced vinegar dip.