The opposing theory states that there are different kinds of receptors for different compounds within a molecule.  One point in favor of the vibration theory is that the different isotopes of hydrogen have different vibrations.  Apparently, there is a difference in the smell for some of the molecules that have one isotope instead of another, which supports the vibration theory instead of the “standard” theory because a receptor shouldn’t really be able to tell the difference.  There’s still debate on which is right, though.