In a 9-by-13-inch baking dish, whisk together the mirin, soy sauce, rice wine vinegar, brown sugar, sesame oil, scallions, 1/4 cup of the vegetable oil and two-thirds of the garlic and ginger. Put the pork tenderloins in the marinade and turn to coat. Cover and marinate for at least 2 hours and up to overnight.
Heat a grill pan over medium-high heat.
Remove the pork from the marinade, allowing the excess to drip off. Transfer the marinade to a small saucepan and bring to a boil over high heat. Boil to reduce slightly, about 5 minutes. Set aside to cool.
Meanwhile, grill the pork, turning occasionally, until a meat thermometer inserted into the center of the tenderloins reaches 145 degrees F, 10 to 12 minutes. Remove to a cutting board and allow to rest for 10 minutes before slicing.
Heat the remaining 1 tablespoon vegetable oil in a large skillet over medium-high heat. Add the remaining ginger and garlic and cook until fragrant, about 30 seconds. Add the bok choy along with 2 tablespoons water and cover. Steam until the bok choy is bright green and crisp-tender, 5 to 6 minutes. Serve alongside the sliced pork with the marinade sauce.