The Boeing QRH then (and now) states that in the event of vibration to retard the thrust lever until the vibration ceases (if necessary to idle). If the vibration continues (highly unlikely at idle) Boeing stated "other engine problems may be indicated".
So, in my opinion, it was procedurally incorrect to shut the engine down in the first place, given that the engine was misidentified.
But like the holes in the proverbially cheese model there were many factors in the Keqworth accident. What about the lack of certification of the engine in that airframe type - engines shouldnt throw blades! But in our business sometimes s**t happens!
It was somewhat ironic that the failure occurred when at height over EMA. If they had been in the cruise on their way to Tenerife over the water and shutdown the wrong engine they would have attempted to stabilise at a lower level, increased the thrust on the offending engine, recognised the error and then had time for a relight on the good engine.
"Mistakes" were made that night but to some extent this accident was a result of the "system" and culture existing at the time.