Combine the vinegar, soy sauce, fish sauce, sugar, red pepper flakes and garlic in a medium bowl and stir until the sugar is dissolved. Transfer 1/4 cup sauce to a small bowl and set aside for serving.
Combine the pork chops and remaining sauce in a 1-gallon zip-top bag. Squeeze in the lemon juice and add the squeezed lemon halves; seal the bag. Let marinate 20 minutes at room temperature, massaging the pork every 5 minutes. Toss the scallions with the vegetable oil on a plate and season with salt and pepper.
Meanwhile, cook the rice as the label directs. Preheat a grill or grill pan to medium high.
Remove the pork chops from the marinade, pat dry and season with salt and pepper. Oil the grill or grill pan, add the pork and grill until just cooked through, 3 to 4 minutes per side. Transfer to a plate to rest. Grill the scallions, turning, until marked and wilted, 1 to 2 minutes. Divide the rice, scallions and pork chops among plates. Serve with the reserved sauce.