They collect fruits and vegetables for the next time Vaal "signals" that he needs sustenance. I really don't think they would do anything unless told to by Vaal, that is what made the society so shocking to our heroes.
Insofar as to how well it worked for them, well that is debatable. If they lack the ability to think independantly, can they really know if something works for them or not. I just don't think they knew (or could know--and that is the point) anything else until their encounter with Kirk and the rest.
Which is where you can make the argument that they weren't protected by the Prime Directive as they are an arrested culture.
The arrested argument is something Kirk came up with, its not part of the Prime Directive, and it's highly debatable in every aspect. When even the episode writers/producers make an anology for our heroes with the devil, you KNOW something is WRONG.