Plantinga talks about what our vocation in God's Kingdom is in the fifth chapter of Engaging God's World. It seems that he would agree with Lewis in that if we wait until our life is "normal" or until the pressures of life have gone away, we will never accomplish anything for the Kingdom of God. There is always going to be something that needs to be done or some pressure that is weighing you down. It takes perserverance to carry on and do God's bidding even in all of these times. Obedience is key.
Plantinga talks about the difference between a "good citizen" and a "prime citizen". A good citizen accepts the commission of Jesus and likes the Kingdom of God. A prime citizen, however, accepts the commission of Jesus with enthusiasm and yearns for the Kingdom of God. The heart of a prime citizen loves the things of the spirit (repentence, forgiveness, etc.) and gets excited when any of these things come to mind. In short, "He or she is a person with a calling." We all have a calling in God's Kingdom. Whatever that vocation may be, not only missionaries or pastors, it can be used to advance the Kingdom of God.