When creating a crosshair plug in I need two vertices. But axCVDisplay1.GetOverlayObjectPosition always returns coordinate (0, 0) for the second vertex 
This is correct. Although the Crosshair plugin requires two vertices for its definition (one for position and one defining the size of the two bars of the cross) it uses only one vertex for interaction with the user. Changing or querying the bar size after the plugin has been created is not supported.