Viaan as a character is the one which every women would like to hate, but at the same time Adnan is portraying it in such a way that we find him comical and cute. I don't know if it is intentional by the makers to make him comical that audience won't be able to hate him after all these deeds. Katha is the super strongest character, if you think about a woman going for ONS, for whatever reasons is accepted by audience in Indian sub continent, its a great achievement by makers and Aditi for portraying her in a highly acceptable manner after ONS.
I am sure Viaan is a lonely soul, and he had some unfortunate and sour relationships in the past which makes him always club money with physical intimacy.
The actual story and real test for Viaan will come after truth about first ONS is out