- Cold-Blooded Logic - The immutable, alien logic of the Lizardmen means they will only consider strong, emotive states when engaging in diplomacy.
The Defender's of the Great Plan receive a solid garrison for the settlements they control. They will be able to fight off most armies that aren't a full stack or of low quality troops. However, if the enemies are a full stack of mid tier quality and above, their settlements will fall. How much you care about this as Nakai is up to you, the only way it affects your campaign is losing the income they provide and the temples that contribute to the Old One's Favour.
One should probably not care too much about the Defenders as they serve only to occupy settlements and provide temple buffs. Perform the rite that gives them a Blessed Spawning army so they can protect your income, but otherwise continue conquering and don't worry about doubling back to recapture a Defender's settlement.
The Defenders will never spend their money or recruit armies of its own. Every few turns, Nakai should be initiating diplomacy to Demand Payment from them to supplement the vassal's tribute.
Nakai is obligated to declare war on any faction that attacks his vassal. Likewise, the Defenders must go to war with whoever Nakai declares against or is declared on.