The following comes from the Paths of Love:
For Pope John Paul II, vocation is fundamentally something personal that takes place between a person and God. Yet there is still a certain "objectivity" to vocation; for being a human person means being oriented to the truth, and truth is objective.
In the first place, then, a vocation is that which God uses to direct each and every one uf us to his task in life. "Jesus has a specific task in life for each and every one of us. Each one of us is hand-picked, called by name by Jesus! There is no one among us who does not have a divine vocation!" Some are called audibly by God, but the usual kind of call is internal, through the inner working of the Spirit. "What is a vocation? It is an interior call of grace, which falls into the soul like a seed, to mature within it."
We cannot give an everywhere valid account of how a vocation takes place, since "apart from the universal elements that are found in every vocation, each call takes place concretely in ways that are always new and always different—and let us add, always beautiful and wonderful, because God is always wonderful in all that he does." But we can give a general picture, as the Pope does in several places.
Do not be slow to answer the Lord’s call! From the passage of the Book of Exodus read to us in this Mass we can learn how the Lord acts in every vocation (cf. Ex 3:1–6, 9–12). First, he provokes a new awareness of his presence—the burning bush. When we begin to show an interest he calls us by name. When our answer becomes more specific and like Moses we say: "Here I am" (cf. v. 4), then he reveals more clearly both himself and his compassionate love for his people in need. Gradually he leads us to discover the practical way in which we should serve him: "I will send you." And usually it is then that fears and doubts come to disturb us and make it more difficult to decide. It is then that we need to hear the Lord’s assurance: "I am with you" (Ex 3:12). Every vocation is a deep personal experience of the truth of these words: "I am with you."