Remember what Vansh did when Ridhima was trying to find out Ragini's truth & the truth abt her parent's accident?
In both occasions Vansh behaved harshly with her , later it was revealed he did all that just to protect her from being more hurt.
So I think Ridhima is also doing the same (specially after that Angre Ridhu convo). She is ready to be even termed as a betrayer, to stop Vansh from seing the big picture & being broken by some bitter truth.