Mix ketchup, vinegar, onion, sugar and Marinade Mix in medium saucepan until well blended. Bring to boil. Reduce heat to low; simmer 10 minutes or until onion is tender. Set aside. Brush chicken lightly with oil.
Grill chicken over medium heat 6 to 8 minutes per side or until cooked through, turning occasionally.
Shred chicken using 2 forks. Add to saucepan; toss to coat well. Spoon shredded chicken mixture on rolls. Serve with Easy Summertime Pickles, if desired.