In 21w05a, the "equip" game event was added. This causes vibrations when armor is right-click-equipped or put onto an armor stand. However, this does not work with items such as saddles and horse armor on horses.
This could also be applied through the mob interact event.
Linked issues
duplicates 1
Comments 2
This appears to be a duplicate of MC-209906, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.
Relates to MC-213915