Riddhima has always gave the impression to Vansh of being a girl that beliefs in relationships and attached to religious customs .She has qualities of being an amazing partner and he has noticed that while keeping his thousand eyes on her .
One thing that is interesting to note that Vansh too emphasises on making her wear sindoor and mangalsutra whenever he gets an opportunity.
His famous dialogues when he informs Riddhima that she is going to marry him -" ye sindoor tumhe meri lambi umar ki dua and mangalsutra tumhe mujhse kabhi dur nhi jane denge . Is ghar se tum tab tak nhi ja sakti jab tak hum dono mein se kisi ki saanse na band ho jaye "
Somewhere give an impression of him trying to claim her despite the fact he wasnt even having much romantic feelings at that point. Yes an attraction was always there from the very begining.
Why ? Why?
The possibility can be that he wants to instill that she belongs only to him in her mind deeply and she can never bring herself to betray him later on . Like a kind of subconconscious manipulation . Exactly like Bashar did with Rudaba by making her say I love U everynight in Bashar Momin and she begins to eventually harbour feelings for him .
We all know that he has a good understanding of human psychology being a mafia and buisnessman .To be honest i dont buy makers track of Vansh believing that Dsouza was a mental patient and wanted to kill him.
Seeing her wearing the marriage symbols kind of satisfy his ego and it can be also that he wants to be with a girl like her .Im sure he has seen something in her that he has decided to marry her infront of Dadi and Sia as he seems logicless to involve family and their emotions .
He knows that she is harmless bcz he never stops her from mingling with both of them and doesnt fear that she might harm them.
He likes her COURAGE to enter his life knowing the consequences. Though She is feels helpless yet doesnt give in to his fear.
So in long run he wants use his enemy's pawn and make her trust him and it looks his plan is working .
Riddhima subconsciously does believe him .
Do let me know ur pov that why so much imp Vansh to this marriage when he knows she is a spy.