I'm an OLD poster, so I had to get a new username, but it's good to be back.
Since the vast majority of vaccines are given in the first year of life, and auditory processing disorders are not diagnosed until years later, it would be awfully difficult to prove a relationship. In fact, that holds true for any autoimmune disorder that is triggered by vaccines--often times, even though they may be triggered immediately, they might not cause noticeable symptoms until they've been active for quite a while. And by that time, it doesn't seem like a cause-and-effect situation, because so much time has gone by.
I do know someone whose daughter became deaf in one ear after her kindergarten MMR, but that's not the same thing as auditory processing disorder.
Not sure how reliable/unreliable hair testing is.
If your hair shows high levels of mercury, it means that there has been exposure--but it doesn't tell you how much, or how much was/was not excreted, or how much may have crossed the blood/brain barrier or gone into organs. And your hair DOESN'T show high levels of mercury, it doesn't mean you haven't been exposed.