Immerse the skewers before lighting the grill, so they have plenty of time to soak. Prepare a medium-hot fire in a charcoal grill or preheat a gas grill on medium-high.
Remove the tenderloins from the refrigerator 20 to 30 minutes before grilling. Cut the pork on a sharp diagonal, across the grain, into 1/8-inch-thick slices and place in a large bowl.
Cut off and discard the dried, grasslike top half of the lemongrass. Trim the base from the bulb end and remove and discard the tough outer leaves. Using only the white and light green parts, cut in half lengthwise, and then cut crosswise into ½-inch pieces.
In a food processor fitted with the metal blade, combine the lemongrass, ginger, shallot, sugar, salt, cumin, coriander, and red pepper flakes and process until finely minced. With the machine running, add the vegetable oil through the feed tube and process until a paste forms. Transfer the paste to the bowl with the pork and brush or rub the paste evenly over the meat.
Thread the pork onto the skewers, dividing it evenly and weaving each slice to pierce it 2 or 3 times. Bunch the meat a bit, so it covers about 8 inches of each skewer.
Oil the grill grate. Fold a footlong piece of aluminum foil in half lengthwise and lay it on the grill grate. Arrange the skewers so the exposed bamboo is protected from the flame by the foil and the meat is directly over the fire. Use 2 pieces of foil if necessary. Grill the skewers, turning them once, until the pork is cooked through and the edges are slightly caramelized and charred, about 3 minutes per side. Serve immediately.