public enum VertexSymbolType : System.Enum, System.IComparable, System.IConvertible, System.IFormattable, System.ISpanFormattable  

{
  var options = ApplicationOptions.EditingOptions;
  await QueuedTask.Run(() =>
  {
    //There are 4 vertex symbol settings - selected, unselected and the
    //current vertex selected and unselected.
    var reg_select = options.GetVertexSymbolOptions(VertexSymbolType.RegularSelected);
    var reg_unsel = options.GetVertexSymbolOptions(VertexSymbolType.RegularUnselected);
    var curr_sel = options.GetVertexSymbolOptions(VertexSymbolType.CurrentSelected);
    var curr_unsel = options.GetVertexSymbolOptions(VertexSymbolType.CurrentUnselected);

    //to convert the options to a symbol use
    //GetPointSymbol
    var reg_sel_pt_symbol = reg_select.GetPointSymbol();
    //ditto for reg_unsel, curr_sel, curr_unsel
  });
}

{
  var options = ApplicationOptions.EditingOptions;
  //var options = ApplicationOptions.EditingOptions;
  await QueuedTask.Run(() =>
  {
    //change the regular unselected vertex symbol
    //default is a green, hollow, square, 5pts. Change to
    //Blue outline diamond, 10 pts
    var vertexSymbol = new VertexSymbolOptions(VertexSymbolType.RegularUnselected);
    vertexSymbol.OutlineColor = ColorFactory.Instance.BlueRGB;
    vertexSymbol.MarkerType = VertexMarkerType.Diamond;
    vertexSymbol.Size = 10;

    //Are these valid?
    if (options.CanSetVertexSymbolOptions(
         VertexSymbolType.RegularUnselected, vertexSymbol))
    {
      //apply them
      options.SetVertexSymbolOptions(VertexSymbolType.RegularUnselected, vertexSymbol);
    }
  });
}

{
  var options = ApplicationOptions.EditingOptions;
  //var options = ApplicationOptions.EditingOptions;
  await QueuedTask.Run(() =>
  {
    //ditto for reg selected and current selected, unselected
    var def_reg_unsel =
      options.GetDefaultVertexSymbolOptions(VertexSymbolType.RegularUnselected);
    //apply default
    options.SetVertexSymbolOptions(
      VertexSymbolType.RegularUnselected, def_reg_unsel);
  });
}