The Vexs continue to try to use their abilities to control time only to fail in their plans do to the guardians. Yea, they have kill a lot of guardians but Siva damn near wipe out almost all of the Iron Lords on it's own. The Iron Lords didn't even know they could die until they fought against Siva. The only way that Siva was stop was by containing it because the Iron lords couldn't destroy it. Just imagine if the Vexs ever gets their metal hands on Siva, they would be almost unstoppable. If they could control Siva not only they would become more powerful but their number would grow out of control too. Siva could bring back all of the important Vexs that we destroyed and dead guardians to fight for them.