Even if Akamana could defeat Vel'Asha, and there was no guarantee that she could, with the current shape of her forces, how could they handle 3,000 Yakshini nobles fresh out of the higher planes? Let's double down on a miracle and assume that they somehow won regardless. How much of the Angra Theocracy would remain after that clash?
Akamana had many enemies. Menaka could afford to antagonize Vel'Asha because she was Menaka, a dragoness who could count Dark Preceptors, Gods and Sage Kings among her relatives. But Akamana was her own background and had to consider the big picture—not only for herself but for her subordinates as well. And going against the ruler of the Yaksha race at her current level was both foolish and suicidal.