Just caught up with the episode.
I guess even if we had a ounce of doubt Vihaan being Vansh, it got cleared when he came from behind and told Riddhima tum meri thi air meri hi rahogi.
The entire episode threw lots of hints for us of Vihaan being Vansh. Starting from their fight where he gave an earful to Riddhima for being a deceiver to that bed sharing scene where he plain simply refused to let Riddhima sleep on the couch.
I guess Vansh is fully giving benefits of doubt to Riddhima. He might have learnt something before / after the cliff scene. There has to be a catch.
He is trying to understand Riddhima what she is upto. Vansh always took pride in understanding people. But for Riddhima, she was always a puzzle to him. Maybe because Riddhima always had a fear of him . Now posing as Vihaan, a carefree friend he is testing the waters. Whether Riddhima is actually a betrayer or not. And the equation between her and Kabir. Why she hated Kabir so much if she was his accomplice.
Vansh is taking no risk this time. He will judge first, then will take action. If Riddhima turns out to be faulty he will punish her in his own way. If not, until then no one can even dare to put a claim on her... even for a namesake.
She belonged to Vansh, and always will.