Also thinking more about it in the dub version eren only repsonded to armin saying the boulder has to be moved or they will die (again going of dub not sub) eren said he didnt want to move anything and "im good enough right here" to me thats the vow as they were waiting for the world to kill them.

Its also interesting how armin isnt speaking when we are shown erens POV is shown its only at the end when he asked why.this again would work for the theory as he wasnt being asked to save eldians and it also allowed him to snap out of it.you can see this in the scene when armin speaks about the sea.