Did you ever find your vibration problem? What year and hours are on your mower?
As far as the set screws go, BB uses collars w/set screws for spacers on several spots on the mower and they don't tighten the set screws (because they act as spacers only) and the screws come but, the collars on the spindles and the all the pulley set screws, should be tight!
I hope you ordered an O.E.M. belt from Bad Boy?