All things said, I'm not sure people even understand why they are a vassal here.
Around when we picked Beg them for their wisdom we had seconded decisionmaking to them.
Then we formalized the relationship by sending our leaders to ask them for advice.
Which gave our leaders double the authority but made them very informal liege lords.
Counting from the point where we chose to beg them for their wisdom rather than spank them and drag them home, or cut them loose our choices were:
-Individuals - This puts them as a decentralized source of influence. They would have grassroots influence on whoever felt concerned enough to visit, but no overt leadership level benefits and costs. They may take over, or the difficulty of travel disrupts things eventually. Down this route, if background events were the same, we probably would not have had gotten food in time for the starvation crisis.
-Hunters - This gives them access to our warrior class, which means that we now have Crafter-Leaders and Warriors advised by the shamans. Rather obvious side effects if they manage to sway the warriors to their side and the crafters disagree.
-Crafters - This makes our leaders the intermediary between the people and the shamans. It controls the flow of information, since authority flows from the stones, to the shamans to the crafters to the people. But it subordinates us to them, while strengthening our own leaders control over our people.
-Student - This makes them the petitioner, where they keep a messenger in our camp to tell them whats going on. Down this route we're in control because they send us advisors and petitioners, but they control the flow of information.
-All three - Well, for starters leaving the village without ANY authority figures whenever we consult them is going to be trouble.
Keeping in mind that vassalization is not really a problem as long as we sustain growth, their location is too food scarce to outgrow us, so we'd eat them more than the other way.
Chestbursting is a thing for more complex societies, but there isn't really any way for them to keep us save for continuing to be useful. If one of their quests go wrong...well tables turn.