I am not sure I would call it a vocation, exactly. At least, it is not like these people are pursuing something as a job. But if you are using the term "vocation" in the sense that a priest or a nun is said to have a vocation -- as a higher calling to do something, I guess it makes sense.
Anyway, what these people are doing is wandering around until (they hope) they get a chance to reform society. They have each memorized a book or a part of a book. This is their vocation -- to keep that book "alive" until they get the chance to share that book with their society after their society has changed enough to allow that.
So I suppose their vocation is memorizing books in an attempt to have the chance to renew and remake their society some time in the future.