Sorry if the reply is really late......but I have more info.
Them invades Vayroia because there is no widespread human like civilisation on Vayroia. So he assumes that the inhabitants on Vayroia are soulless, unintelligent and so he assumes that their world will be easy to take over.
Them is is wrong because the inhabitants of Vayroia, the Vayamon, have personalities and feelings. It is a struggle, but one group of Vayamon eventually get rid of Them and the rest of the Ilorians..... For now......