this.positionVector = this.playerSpawn.positionVector;
This seems to be your problem line. It is indeed a reference; try calling
this.positionVector = this.playerSpawn.positionVector.Clone();
Edit: Not sure what guarantees Clone() makes, so if that fails, try:
Vector2D vector = this.playerSpawn.positionVector;
this.positionVector = new Vector2D(vector.X, vector.Y);
The latter will work for sure.