Vidya never ever let your enemy know you know their plan of attack, that you're about to expose them or show them any mercy.
I don't believe Vidya or Sagar can be so dense & thick in the head. Vidya as soon as Aunt leaves so does Vidya brain cells, she gets into fight with Sindoora & gets knocked out leading Sindoora to find out that she is NOT pregnant at all which Sindoora uses against her. But the worst thing was to write that note confessing about the non pregnancy. (That dream sequence with Sagar finding that note was scary!!)
& what about Sagar how can he be so in the dark about Sindoora. The phone call Vidya made letting Sagar hear everthing Sindoora said should've snapped him out of it, but no he still trusts that heartless witch over Vidya.
So what's it gonna to take to take down Sindoora?
For Vidya to never let Sindoora or Sagar see her coming!!