What 's the different between vector2 and vector3 assigned by Vector2.lerp?

When I coded different form of assigned transform.right, I found I got different result.

  Vector2 vecRes = Vector2.Lerp(transRight, dirNorm, myManager.rotationLerp);
  transform.right = vecRes;

has the right result,but

 transform.right = Vector2.Lerp(transRight, dirNorm, myManager.rotationLerp);

has the wrong result.

Should I assign a new Vector2 var then to assign the new var to transform.right? Or should I have to (Vector2)Vector2.lerp() first？

Edit: I should not use Vector2.lerp to process rotation then assigned to transform.right. transform.right will use quaterion * vector3.right get the result. I should use Quaternion.Lerp() to get rotation then set to transform.rotation.