My thoughts were exactly similar.Fatal attraction and Glen Close came to my mind too when I watched the pre cap in Friday's episode.
If Mandira had been graceful and dignified in her loss Vijay would have found it very difficult to get over her and move on.Now with this behaviour she is making it easy for Vijay to move on with Bulbul.
But I think she always managed to get what ever she wanted till now. This is the first time the object of her affection moved out of her grasp and she is not taking it well.
She probably would have managed to extricate herself from her engagement with Anant and gotten back to Vijay. But Vijay had closed the door on their relationship by getting married to Bulbul. So there is anger against Vijay too.
Another reason could be that she is spoilt by her father.I think he gave her everything she wanted till the time she wanted Vijay as her husband.At that time she went against her parents and now Vijay is denying her what she wants. Bulbul is in her way so she will not spare her too.
But I so want her to give up the idea of marrying Vijay and make things difficult for him in other areas of life and that would be more interesting.