Bring vinegar to a boil over high heat in small saucepan. Boil 5 minutes or until vinegar is reduced to about 2 tablespoons.
Meanwhile, evenly coat chicken with 4 tablespoons flour.
Melt 2 tablespoons I Can't Believe It's Not Butter!® Spread in 12-inch skillet over medium-high heat and cook chicken, turning once, 8 minutes or until chicken is almost done. Remove chicken from skillet and set aside.
Melt remaining 1 tablespoon Spread in same skillet over medium heat and cook onion and mushrooms, stirring occasionally, 6 minutes or until vegetables are tender. Stir in garlic and cook 1 minute. Sprinkle with remaining 1 tablespoon flour and cook, stirring constantly, 1 minute. Stir in reduced vinegar and broth, scraping brown bits from bottom of skillet. Bring to a boil over high heat. Reduce heat to low and return chicken to skillet. Simmer 2 minutes or until chicken is thoroughly cooked and sauce is slightly thickened. Sprinkle with basil and season, if desired, with freshly ground black pepper.