public class VertexSymbolOptions 

//Custom tools have the ability to change the symbology used when sketching a new feature. 
//Both the Sketch Segment Symbol and the Vertex Symbol can be modified using the correct set method. 
//This is set in the activate method for the tool.
protected override Task OnToolActivateAsync(bool active)
{
  QueuedTask.Run(() =>
  {
    //Getting the current symbology options of the segment
    var segmentOptions = GetSketchSegmentSymbolOptions();
    //Modifying the primary and secondary color and the width of the segment symbology options
    var deepPurple = new CIMRGBColor() { R = 75, G = 0, B = 110 };
    segmentOptions.PrimaryColor = deepPurple;
    segmentOptions.Width = 4;
    segmentOptions.HasSecondaryColor = true;
    var pink = new CIMRGBColor() { R = 219, G = 48, B = 130 };
    segmentOptions.SecondaryColor = pink;
    //Creating a new vertex symbol options instance with the values you want
    var vertexOptions = new VertexSymbolOptions(VertexSymbolType.RegularUnselected);
    var yellow = new CIMRGBColor() { R = 255, G = 215, B = 0 };
    var purple = new CIMRGBColor() { R = 148, G = 0, B = 211 };
    vertexOptions.AngleRotation = 45;
    vertexOptions.Color = yellow;
    vertexOptions.MarkerType = VertexMarkerType.Star;
    vertexOptions.OutlineColor = purple;
    vertexOptions.OutlineWidth = 3;
    vertexOptions.Size = 5;

    //Setting the value of the segment symbol options
    SetSketchSegmentSymbolOptions(segmentOptions);
    //Setting the value of the vertex symbol options of the regular unselected vertices using the vertexOptions instance created above.
    SetSketchVertexSymbolOptions(VertexSymbolType.RegularUnselected, vertexOptions);
  });

  return base.OnToolActivateAsync(active);
}

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);
});