In a large saucepan over medium-high heat, warm the oil. Add the onion and garlic and sauté until softened, about 3 minutes. Add the pork and cook until it turns white, about 5 minutes. Add 1 cup water and bring to a simmer. Add the ubod, sugar, and fish sauce and cook until the ubod is tender, about 5 minutes. Add the shrimp and cook for another minute. Taste and add more fish sauce if needed. Set aside to cool.
To assemble, place a wrapper on a flat work surface. Spread about 2 tablespoons of the sauce over the wrapper. Top with about ¼ cup of the filling and place a scallion half over the filling, extending past the outer edge of the wrapper. Fold the bottom of the wrapper up over the mixture. Fold in the sides so they overlap. Serve at room temperature.