I use the Flutter INSTEAD of the Vest. I have an Acapella but never wanted to use it because it was hard to clean -- now the Acapella Choice model can be boiled.
I use the Flutter after Hypertonic Saline. I do it for about a half hour and the TWO together virtually clears my lungs of mucus. I like the Flutter because of its portability. I can do it in the car, which is a great timesaver.
When I used the Flutter alone, I was still getting frequent exacerbations, but COMBINED with H.S., wow! Big difference that I noticed almost immediately.
The Flutter can be found online for a little less than $60. I also recently bought the Acapella Choice very cheaply (haven't used it yet). I can look up the websites if you'd like. Insurance usually covers these devices.
But for 60 bucks, you've got nothing to lose. And I'm guessing since you have the Vest already, and your doctor is recommending the Flutter -- your doctor would like to see you get better mucus clearance.