I hate that insurance companies think they know more than doctors. I'm not a doctor, but obviously your doctor prescribed the vest for you, and he/she knows you've had the bleeds. It's frustrating that the ins is denying it, but it's also quite common.
Your clinic should know how to advocate for you in this matter. I'd ask them to write a letter for you. Airway clearance is necessary to prevent/lessen infections that your ins. company will have to cover meds/hospital stays for.
As far as the vest being harmful in regards to bleeds: Every patient and every bleed is different. Ask your doctor this question, because he/she knows your particular situation, and can better answer you.
Stacey