Kunal and Sonia, I feel their relationship broke because Kunal could'nt go against his Dad ever and stand up for her even though he loved her.. Every time Sonia confronts Vandana, she mentions this.. Maybe this was the main and only reason she betrayed Kunal as she was getting all the attention and luxuries when she was with Kunal too.. Kunal says he loved Sonia dearly, still couldn't get over the break-up and can never love anyone else, but I guess he couldn't give the respect Sonia deserved as his wife and daughter-in-law of the house when they were married.. We all can see how his Dad treats Vedika, his own daughter, and can imagine how he must have treated Sonia whom he never liked.. Kunal loves Vedika too, but never went against his Dad for treating her like this all these years..
Now we have to see how Kunal changes with his love towards Vandana.. whether he'll be able to keep his promise to Vandana's Dad that she will get all the respect she deserves as his wife and daughter-in-law of the family...and, Vandana is strong and demands respect and can stand for herself whereas Sonia needs someone else to support her.. this change in characters between them can make a huge impact too in their relationship and Kunal may get the courage to speak up against his Dad for what is right and wrong...