Preheat the grill to medium heat. Cut the bok choy in half lengthwise. Combine the soy sauce, sesame oil, and rice vinegar in a small bowl.
Brush the cut side with this mixture. Place on the grill and close the grill. Cook for 3-4 minutes until beginning to char and become tender.
Once the bok choy is caramelized with some grill marks, flip and carefully brush with sauce again. Cook for an additional 3-4 minutes.
Remove from grill and drizzle with sweet chili sauce (optional).
