There is a difference between vibration indications which are displayed on a gage within the cockpit and vibration which is suddenly felt in the cockpit. Vibration gages alone are not reliable indicators of an engine malfunction anymore than fire detectors in a cargo hold a reliable indicator of a fire. For a variety of reasons associated with the gage and detection means, false indications are common. Thus it's important to follow recommended procedures regarding the engine to ascertain the extent of the problem before giving up on the engine and turning off not just the thrust but the accessories that go with it.
lomapaseo, yes I agree. If I recall correctly one of the issues was that the vib indicators on older engines were unreliable (when I was on the B707 I remember the flight engineers saying they were a waste of time!) but the vib indicators on later a/c are much more reliable. This was not covered in training. Obviously all factors should be taken into account before shutting an engine down (easy to say when you are on terra firma but when you are in the hot seat.....)
Regarding the engine certification I recall that the thrust on the engines had been increased but the full certification process had not been applied in this case with respect to that engine being in that a/c. Maybe I recall incorrectly but my basic point is that modern engines shouldnt lose blades and that this was part of the error chain. You could say another part of the chain was the mickey mouse vib guages fitted to the early B737-300s.