AutoCAD .NET :: Draw Triangle In Segment Of Polyline In Segment Direction

I want to draw triangle in segment of polyline in segment direction.

I have target segment and center point of it:

  Polyline line = ...
  LineSegment3d centerSegment = line.GetLineSegmentAt(centerSegmentIndex);
  Point3d centerPosition = centerSegment.MidPoint;

I can find direction of this segment:

  Vector3d zaDirection = line.GetLineSegmentAt(centerSegmentIndex).Direction;

Next, I create list of points:

  List<Point3d> points = new List<Point3d>();
  points.Add(centerPosition);
  points.Add(centerPosition.Add(new Vector3d(ZASize, +ZASize, 0)));
  points.Add(centerPosition.Add(new Vector3d(ZASize, -ZASize, 0)));

And draw it:

wd.Geometry.Polygon(new Point3dCollection(points.ToArray()));

Ok, I have triangle but... it's not in segment direction, it's X-direction.

I try to rotate it:

points.Add(centerPosition.Add(new Vector3d(ZASize, +ZASize, 0)).RotateBy(0, zaDirection, centerPosition));

But it's not the same as I want.

How I can draw triangle by serment direction?