At first I didn’t understand why Armaan would be adopted but I think I know why now.
I’m guessing that Vidya was unable to conceive as the doctor said that she couldn’t become pregnant (like Akshu/Nandini) and so her and Madhav decided to adopt in order to give the Poddars an heir to please Dadisa.
But miraculously she became pregnant later on (like Akshu/Nandini) and had Rohit and so Cruella Dadisa loves him more as he has actually Poddar blood unlike Armaan but Vidya still loves him as he is still technically her first child.
Aage Ki Kahaani Jaane Radha Rani