My cup object's inside vertices is slipping through like this

I already have recalculate normal, check if there is overlapping vertices or not, and when i zoom in, the image's vertices just like in above picture but when i zoom out, it is fine, there is no vertices slipping like in the picture. I even use merge tool, but there is no by distance function in the tool unlike in the blender 2.8 version when you get one to merge the vertices.
How to fix this mess?
