I think that Vasu was holding back her gussa until now........coz from yesterday's preview....it looked like she was saying Sadhna....
"tumhari himmat kaise hui hamse shart rakhne ki..."
I think Vasu had this grudge against Sadhna for asking Ranvir to marry Ragini.....She surely wanted to show Sadhna "who is the boss of the house"......and that nothing or no one can change anything in the house and take away her powers to rule the house.
coz when u come to think of it....she has not only destroyed RagVir's relationship into pieces so that they can never get back together....she has also tried to show her...her place in the house by making Alekh against her.....by using the same shart topic....so that now even Alekh will neglect and disrespect her.