Vibration is produced by the electromagnetic forces between the current-carrying conductors that make up the end-winding. The forces are proportional to the square of the current, with currents in the same direction causing attraction, and current in opposite directions causing repulsion.
The vibration can be reduced by improved support and bracing the end-winding, and particularly ensuring that the end-winding does not have natural / resonant vibration frequencies at or close to supply frequency or multiples thereof. Obviously reducing the current in the winding will reduce the vibration but load reduction is mostly not an option.
It is important to determine the root cause of the cracks, in order to have the best chance of preventing them in future. The most likely cause of end-winding cracks is a FAULT on the alternator. Any transient fault that causes high currents in the stator will cause very large shock forces on the end-windings.
Obviously natural frequencies close to the power frequency or its close integer multiples will make the fault stress much greater - increasing the chances of cracks. You can actually perform a bump test and check this yourself. Use a smartphone with a vibration app and check where the frequency components are. Most smartphones measure up to 100 Hz.
Although thermal expansion can of course cause cracking in the insulation around a copper (or other metal) stator bar - this cracking will tend to occur at ANY BENDS in the bar - not where the bar is braced or supported. I am not an expert in this field - and there are others with much more experience than me - but I am not aware of many cases of stator bar cracks on high voltage alternators due to differential thermal expansion on Resin Rich or individually Vacuum Impregnated bar machines.
Many OEM's and service providers have improved the end-winding supports and bracing of large stators, with good success. I also know of at least one case where the attempts to improve the support has not made a meaningful difference.
Bottom line: Identify what the root cause of the cracks is first, and then you will have a better idea of whether you will be able to prevent them recurring.