I used the vest for many years. I was probably one of the first users of the vest when it came out. I found that if you huff out your cough during the session it will help. When my CF progressed, i used the Susanne's Somers thigh master along with vesting to aid in my coughing. It helped to strengthen my chest and arm muscles while coughing. I would cough up lots of goodies.
when I was waiting for my first transplant, I did four CPT therapies and after my therapist would leave, I would use my vest for fifteen minutes. Thankfully, I did all this because I still progressed and I cannot imagine how I would have survived if i did not bring up all that mucus.
Drink water in small sips to see if that helps.