While not an authoritative source for such information, it is worth noting that even Answers.com can have something useful to say about it.
Answer: To have a vocation is to have a specific “calling” or disposition towards the religious life. This means that God has granted the soul the grace of a vocation, or the invitation to intimately participate in ministry of His Church or to live, pray and work closer to Himself by living a life that is removed from the world.
A vocation to the priesthood is a specific calling a young man feels to enter a seminary in order to train to become a priest. This calling may be simply a desire to serve as a priest, an intellectual resolution based on self evaluation, a religious experience (though these are rare and not to be looked for), or a recommendation by a parish priest or retreat master.
A vocation to the priesthood is confirmed when the candidate enters the religious state and when the Church calls him to approach for ordination.