I am surprised that you weren't given a recommended set of speeds/pressures to use when you received the vest from her clinic dr or pt... it may have been an oversight so I would call them.
In our case, we were given a set of 5 speeds (10-14hz) that we used for a while but then I was interested in trying Dr. Warwick's recommendations. (He invented the vest & has done studies on effectiveness w/ several types of vest.) Anyone interested in that can find info about on Amy's (savingferris) blog; scroll down to her Warwick link. We use speeds of 6, 8, 9, 18 and 19 and start with a higher pressure (in Emily's case the highest she can tolerate is 8) and it gradually goes down as the frequency increases.
As others have posted, it's important to vary the speeds and pressures no matter how you end up doing it, for the maximum benefit to both the small and large airways.