I read on it years ago, but I think it helps kill the mold spores.

"It's not exactly that vinegar itself extends the life of berries. It's the fact that vinegar is so acidic that it kills or inhibits the growth of a lot of the bacteria and fungus, including mold, that may grow on berries, which makes the fruit last longer," explains Sean Brady Kenniff, EatingWell's senior digital food editor. (By the way, this same technique should work to clean just about any fruit, not just berries.)