Preheat a grill to medium high. Cook the rice as the label directs; set aside. Meanwhile, brush the red onion and pork with 1 tablespoon vegetable oil; season with salt and pepper. Mix the plum sauce, 1 tablespoon vinegar, 1 teaspoon chili sesame oil and the ginger in a small bowl; remove half the sauce to a separate bowl and set aside.
Oil the grill grates. Grill the red onion and pork, covered and turning occasionally, until the onion is tender, about 10 minutes; remove the onion to a plate. Brush the pork with the plum sauce mixture and continue grilling, covered, until a thermometer inserted into the middle registers 140 degrees F to 145 degrees F, 5 to 10 more minutes. Let rest 5 minutes, then slice.
Meanwhile, put the bok choy in a large microwave-safe bowl, cover and microwave until crisp-tender, 2 to 4 minutes. Drain and toss with the remaining 1 tablespoon vegetable oil; season with salt and pepper. Grill, turning, until charred, 2 to 3 minutes. Roughly chop the bok choy and onion.
Fluff the rice; divide among plates. Top with the vegetables and serve with the pork. Stir the remaining 1 tablespoon vinegar and 1 teaspoon chili sesame oil into the reserved sauce. Spoon over the meat and vegetables; top with sliced scallions.