Dadi being black mamba doesn't make much sense to me. Firstly, because she doesn't need to hire an assassin to kill Vansh, she can kill him herself openly and no one in the family would even question it.
Secondly, unless it's all an act that she is putting on for the family, it actually does seem like she cares for vansh. Got all senti looking at their photo, and is most worried about how Vansh would react when her kaand is exposed.
Thirdly, Sara came to kill Riddhu when V and R were running away from Chang's men in the jungle. That means the order to kill Vansh had been given before then only. And, there seems to be no reason for dadi to want to kill them then.
Fourth, why would she work for Vyom from the very beginning? She could be blackmailed by him now, but not initially.