Preheat a grill to medium. Halve 1 pound bok choy lengthwise; microwave, covered, 3 to 5 minutes. Whisk 1/4 cup sweet chili sauce with 1 tablespoon rice vinegar and 1 teaspoon each brown sugar, sesame oil and soy sauce. Toss with the bok choy. Grill until charred, 1 to 2 minutes per side, brushing with any extra sauce.