Cook rice according to package instructions.
In a medium bowl, whisk together vinegar, soy sauce, sugar, and ginger until sugar is dissolved. Reserve 2 tablespoons marinade. Add sirloin to the bowl, and let marinate at room temperature about 15 minutes.
Preheat broiler. Brush a rimmed baking sheet with oil and add arrange bok choy on one half of sheet. Whisk 2 teaspoons oil into reserved marinade and drizzle over bok choy. Broil until starting to soften, about 5 minutes. Meanwhile, thread sirloin and scallions onto 4 metal skewers. Flip bok choy, cover with foil, and add skewers to sheet. Broil 4 to 6 minutes more, flipping once. Serve with rice.