Vishaka maybe because she thought that Mahir was the killer of Bella. She also knows full well that Bella and her hesitancy to kill Mahir is a sign that Bella was clearly falling for Mahir bit by bit. Otherwise, Bella wouldn't be so hesitant to kill or even harm Mahir. It could also be because maybe Vishaka had some unrevealed affair with Vikrant. I think Vikrant was in a relationship with Vishaka and pretended to be in love with Bella. Maybe they could come from the same clan and wanted to get the Naagmani together. She knows that Vikrant is alive and will return someday to dupe Bella into getting them the Naagmani and cannot have Bella's love and loyalty towards Vikrant wavered.
Vikrant because he wanted Bella for himself and because he wants to get the Naagmani so that he can make his mother the Naagrani.