[EDIT: I misunderstood your question. What you would like is a way to just get the bezier vertices, not all vertices. Per your direct quote: "All of this is ok, until I want to change a shape that has been made using some bezier vertexs myShape.bezierVertex(...), and not only the typical vertex() method." Sorry sorry sorry. For the sake of the record, here is my totally unhelpful suggestion.]
getVertexCount()