Just to share my little experience, hope this can help someone else.
My Vertex was perfectly tuned since weeks but today, I got layers shitfing (X axis)
Of course, check the tension of belts, pulley, axes alignment… All was correct.
After some observations (motor rotates, head moving was heratic), I discovered the cause of the problem. The pulley on the axe of X motor was loose.
Seems it’s the last 1% of layer shifting problem (see raby’s post : viewtopic.php?f=62&t=15531) 