Bring sugar, butter, milk and marshmallows to boil in large saucepan on medium heat, stirring constantly. Cook 5 min., stirring constantly. Add peanut butter chips and vanilla; cook 2 to 3 min. or until chips are completely melted and mixture is well blended, stirring frequently. Spread onto bottom of prepared pan. Sprinkle with nuts; press lightly into fudge with back of spoon.