I recall that the vibration indicators on the 737-400 were EFIS type (not steam driven as in the 737-200 series). Now if the vibration level gets excessively high and the needle swings up and around to max stop, the design is such that the needle concerned actually disappears from the instrument dial. Rather similar to the glide slope indicator on EFIS aircraft that is blanked from view until it detects a useable GS within certain localiser angles off.
One of the results of this vanishing vibration needle is that the good engine indicated the normal vibration level (low) while the defective engine vibration needle showed nothing because it had taken itself off the air due severe vibration. This was partly why the crew mis-identified which engine had experienced problems.