1. Whisk 2 tbsp each mirin, soy sauce and vinegar, into a bowl. Add chicken. Toss gently and refrigerate 1 hour. In the meanwhile soak 4 bamboo skewers in water.
2. In a saucepan combine remaining mirin, soy sauce and vinegar plus sugar. Bring to a boil over low heat and let simmer until glaze is slightly thickened and glossy, about 15 minutes.
3. Prepare a grill pan to medium-high heat. Lightly oil.
4. Thread chicken cubes and scallions onto skewers. Discard marinade.
5. Grill until just cook throughout, about 10 minutes, turning every 2 to 3 minutes. Brush with glaze as you turn. Brush again in final minute of cooking.