Combine liquid brown sugar and syrup. Cook rapidly for 10 minutes. Stir in vanilla. Pour popcorn into large kettle or Dutch oven. Quickly stir syrup into warmed popcorn. Stir over low heat a few minutes until popcorn sticks together. Shape into balls. Use thin rubber gloves and dip gloved hands into cold water before shaping balls. Wrap in plastic film or cellophane. Tie with yarn or ribbon to hang on tree.