Place the pork tenderloin in a bowl with the shallot, garlic, fish sauce, sugar, black pepper, and oil, if using, and turn to coat. Cover with plastic wrap and marinate, turning occasionally, at least 1 hour and up to overnight in the refrigerator.
Preheat a grill to high. Grill the tenderloin, turning occasionally, until just cooked through and an instant-read thermometer registers an internal temperature of 145 degrees F. Transfer tenderloin to a plate and set aside to cool to room temperature before proceeding. Once cooled, slice tenderloin into 1/8-inch thick slices and set aside, covered and refrigerated, until you are ready to assemble the spring rolls.
Prepare the rice vermicelli according to package directions and strain in a colander. Run under cold running water until cool and set aside to drain.
Assemble the rolls 1 at a time: Dip 1 spring roll wrapper into lukewarm water and quickly transfer to a clean kitchen towel. (Wrapper will soften within seconds.) Lay 4 shrimp halves in a horizontal line across the center of each wrapper and top with a few tablespoons of cooked (well-drained) vermicelli. Top the vermicelli with 1 or 2 slices of cucumber, 2 tablespoons or so of the shredded romaine, 4 mint leaves, 2 sprigs of cilantro, and 4 basil leaves. Place a few sticks of carrot julienne along the top of the filling ingredients and top with 2 slices of pork. Carefully pull the lower edge of the wrapper up and over the filling. Fold the 2 sides inward over the filling and press to seal. Work carefully so as not to tear the wrapper. Once both sides are folded inward over the filling, roll the spring roll upwards so that the filling is tightly contained and roll up to seal. Set the spring roll aside on a plate, covered with a damp paper towel or damp clean kitchen towel. Repeat with the remaining wrappers and filling. Once you have assembled all of the spring rolls, serve them immediately ñ either at room temperature or slightly chilled, with the Peanut Sauce on the side for dipping.
Yield: 4 to 6 servings