Turn your grill on high to heat up for at least 10 minutes.
In a medium pot, stir together the soy sauce, vinegar, sugar, garlic, and ginger. Bring to a boil over high heat, then reduce the heat to medium to maintain a rapid simmer for about 15 minutes, or until reduced by half and the glaze coats the back on a spoon when dipped in.
Cut the white/light green parts of the scallions into 1-inch pieces. Skewer the chicken and scallions (see video).
Transfer about 2-3 tablespoons of the glaze into a separate small bowl and brush it onto the skewers on all sides. Grill for about 4 minutes, or until you have nice grill marks/a bit of charring. If it’s burning too much, turn down the heat. Flip and brush the remaining glaze over top, then cover and grill for about 3-5 more minutes or until cooked through.
See note and "leftovers" section for serving ideas.