I’ve had issues with the Vertex piston denting in in the center and eventually breaking thru. I’ve also ripped a few pins out of them. I like the wiseco 793 piston. Quick break in, strong, and the dome will only start to dent in once the piston is well past its prime and change interval. It will work just fine with your 01.
Try and stay away from the other popular option which is the 2005 CR125 piston. Its just weak all around and you will have to tune back the timing way to much to get it to live.