This .NET class wraps the AcDb3dPolylineVertex ObjectARX class. 

The PolylineVertex3d class represents the vertices within 3D polylines in AutoCAD.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.Entity
        Autodesk.AutoCAD.DatabaseServices.Vertex
            Autodesk.AutoCAD.DatabaseServices.PolylineVertex3d

Public Class PolylineVertex3d
Inherits Vertex

public class PolylineVertex3d : Vertex;

PolylineVertex3d objects contain the Entity methods to set the entity properties such as color, layer, and linetype. However, vertices are subentities that are supposed to have the same entity properties as the parent polyline. Therefore, using the Entity methods within the PolylineVertex3d objects to change these properties to values other than those of the parent will result in corrupt vertices. An audit can fix the corrupt vertices. 

Do not derive from this class. Attempting to do so will cause your application to halt AutoCAD. 

DBObject, Entity, Polyline3d