Just caught up with the last two episodes, and Vaani is giving me all kinds of evil mastermind feels. My main issue with her is that, I could be wrong, but wouldn't all victims of domestic violence, in their fear, try to hide the fact that they are in an abusive relation? So, they would be careful of all the marks in their bodies, and wear clothes that cover it up completely. Here, on two occasions Dev spotted the marks on her body so easily, as if it was meant to be found.
So, my theory is that, the girl with Sahaay is actually Mehak. Remember, during Mehak's death scene, Mehak didn't recognize Dev. She was scared and frantic.
So what if, Mehak belongs to a crime organization (or is the head), the symbol that is popping up is getting pretty frequent now. So, she had married Dev to kill Swami, and after that was done, maybe around that time, she found out about her look-alike (Vaani), and she (using this organization) planned an identity switch. That is the reason, why she had her "freaking out episodes". Then that day, she switched Vaani and her place, and maybe Vaani does have some psychological problem. It got worse when she wakes up at a strange place with a gun in her hand. In the confrontation with Dev, Vaani ends up pulling the trigger and killing herself. Meanwhile, Mehak takes on Vaani's identity. Only to have Mr. Sahaay wake up from his coma. But since Dev was acquitted, she might have thought it was safe to be with Sahaay till Mehak's death controversy dies down.
It somehow looked like, Sahaay was more of a puppet and Vaani as the mastermind. Maybe, she has been manipulating and making him do all that we saw him do. It could be that, she is the one who kills the guy in next week's episode and is framing Dev. Or she could be like Moriarty from Elementary. Seriously feel Dev-Mehak/Vaani track is heading towards the Moriarty-Sherlock track from Elementary.
This would also explain why Meera was not able to contact Mehak.