Vertex edit mode is w3rd. From what I can tell, all other modes move vertecies or whatever based on how far you move your mouse.
I think vertex edit mode doesn't mode the vertecies at all until you are moving you mouse at a certain velocity depending on the grid size.
Try having a grid size 16 or 32 and moving your mouse really slow and they won't move at all, but if you only move your mouse a small way but really quickly then they will move
Also, unlike normal camera mode, vertex edit mode take no notice on how far zoomed in you are in the viewport
