Vocation is a calling from God. This is correct. We are each given a sort of task or role within the church and within society at large. It is part of this grand, overarching plan of the Heavenly Father. But is this role merely a sort of job? Are we just here to do our one job, serving society and the church? No, we have a greater calling as well.
We are also called to restore the earth to shalom. We must take our small role, our minuscule kingdom, and fit it into that great kingdom of God, that overarching plan of God. So how does this affect things? We must be striving to return the world to peace. We must be working with a greater goal in mind. We must heal society and its brokenness.
Does this mean that we are all destined to become "prophets, priests, and kings" or in today's language, teachers, pastors and politicians? Should we all focus on these careers as "righteous" ones? The truth is that we are all teachers, pastors and politicians (in a good sense of that word, forget its negative connotations). We all must teach the gospel. We all must shepherd the church. We all have power and authority in God's kingdom. However, we do not all need to fill those specific jobs within society. We can live our regular lives in such a way that the kingdom of God and the work of shalom is spread through the small, everyday actions we do each day.
(In Plantinga's book, he then goes into depth into the value of a Christian college education. As I feel that I have written a large number of blogs on this topic, I don't think that I will cover it here. )