It is precisely because she has given a lot of thought to how Shashtri ji will react to the truth that she can never be the cause for the truth to come out. This is not a deliberate act of any of the characters parts. All the characters grow and change according to what they are put through. Rads had to go through an initial period of numbness, and pain, and confusion before she herself believed that what happened was not righteous. But by that time things had gone beyond repair. At that point if she revealed the truth everything would be far worse than it was...and that situation is still present. So she can never have the courage to open her mouth and cause the destruction that will follow.
Vish refusing to marry in front of the world would not have been a problem if she did it in an honorable way and not by running away with Purab. Rads stood up in front of the whole crowd and declared properly her reasons for not getting married, and for a conservative communtiy that she lives in the reasons she gave were "honorable". Unlike Vish who wants to become an actress... which for a community like that in Chhoti Bahu's Vrindavan would be scandleous. So its unfair to claim those two situations to be the same.