When Shangguan Hong and the others learned of what Meng Hao was doing, they did nothing to stop him. The Sect Leader contemplated the matter briefly, then chose to ignore it.
As long as Meng Hao's actions didn't affect them or their plans, then he could do as he pleased. As long as he didn't do something to endanger the entire Vast Expanse School... then they didn't care what he did.
Furthermore, Meng Hao was careful in his actions. He only used his own forces, and he only expanded into areas that the Vast Expanse School didn't already control.
As his forces began operations, he chose to go into secluded meditation in Ninth Paragon City, and would only receive sporadic updates regarding affairs on the outside.
Time passed. In the blink of an eye, ten years had gone by.