Put the vinegar, 1/2 cup of the honey, 1 Tbs. of the oil, 2 tsp. salt, and 3/4 tsp. pepper in a 3-quart saucepan. Simmer over medium heat, stirring occasionally, until reduced to 2/3 cup, 13 to 15 minutes. (The glaze should brush on easily but be thick enough to coat the chicken. If it’s too thick, add a few drops of hot water.) Meanwhile, thread the onion, chicken, sweet potato, and peach onto the soaked skewers.