Seriously, they should have shown them lovers but as Samrat's surname is different, she never connected Virat and Samrat as cousins until Haldi.
Thats why when Sharmila mentioned that in Tamil, Sathya is the one who demands the promise from Vikram in return, which made so much more sense than Virat volunteering for that vaada.
But since they made him vaada boy, thye had to make him say it I guess.
Look the vaada was the most stupid and irrelevant thing
Even if he didn’t give her the vaada she would’ve married Samrat because even after he rejected her on haldi, she did come for the sangeet and not even once she said “I will not marry Samrat”