Don't take this the wrong way, I can't stand insurance crap either.
But - in my own experience, I've had a pretty easy time obtaining medical equipment, including the vest. I've had a VERY difficult time getting home healthcare services, such as in home CPT (which would be my preference).
Insurance may have a good point - if the child is too small for the vest, then its too small for the vest... the vest does have some size limitations. Of course, I'd be interested in knowing if the child's clinician recommended it, or just suggested trying it... two very different things. In my experience, clinicians often suggest trying stuff to help patients figure out what they like, which may not carry the same weight as a recommendation, or the physician saying "this child needs the vest."
Personally I agree with the insurance company that CPT is more effective for the kid, but I feel that way for children and adults. I respect the possibilities that the child may be difficult, not like CPT, there may be time/ job issues that make CPT difficult... I also see the possibility that the parents just don't want to deal with giving the child CPT everyday, and see the vest as "an easy way out". This isn't a criticism, I'm just being honest, and if there's one thing I've learned, there isn't a shortcut when it comes to CF. The parents should still press the insurance company - if the insurance company has said that CPT is more appropriate, is it willing to pay for a therapist to come in every day?
But really, if the child is difficult, doesn't like being pounded on, etc., then giving that child a vest that will possibly be placed on a setting that is too low, not used frequently enough, you aren't doing this kid any favors. A CPT therapist would establish a routine, by showing up everyday would likely increase compliance, and would provide much more effective airway clearance.