Vertex painting artifacts

Is there any way to avoid producing “sharp edges” while vertex painting?

Looks to me like you have a problem with your mesh - perhaps those adjacent faces aren’t actually connected or there are additional hidden faces between them. Try selecting all in Edit mode and Remove Doubles.

I am not that bad as a modeler to randomly leave open edges. It is definitely some tool setting or Blender bug. Happens every time when I paint faces at “grazing” angles relatively to view direction, no matter what model I paint on. I have to fix it manually, super annoying. Vertex colors arent even supposed to work like that(

It certainly isn’t supposed to work like that - the vertex should be colored the same for all shared faces. I’ve just tried a few tests at grazing angles can’t can’t get the same sharp cutoff. Is there something else specific to your mesh that could potentially be causing this? Can you replicate it with, say, a plain old cylinder? Similar result on a freshly created cylinder. Maybe “grazing angles” isnt a good term, but “when camera doesnt see the face while painting”. Then, even if the face shares colored vertices, color doesnt propagate to it