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

var options = ApplicationOptions.EditingOptions;

//Must use QueuedTask
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;
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;
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);
});