Dear Bruce,
You ask a very important question that goes to the heart of what a vocation is, what our freedom is, and how God treats us.
If you can, read George Weigel's book on Pope John Paul II called 'Witness to Hope'.On pages 68-70 you will find a brief description of how the Pope as a young man came to understand he had a vocation, what his thoughts were, and how he answered and overcame the initial objections of some people who loved him dearly but tried to dissuade him from the priesthood. I think you will find those pages helpful.
The word vocation comes from the Latin 'to call'. It is God who calls. When we discover a vocation, we discover that God seems to be inviting us to a closer relationship and service, as St Paul explains in the letter you mention. Since original sin has weakened our will and clouded our understanding we find it very hard to understand God, for his call always seems to ask us to do something more difficult, almost impossible, and not what comes easier to us. For example; you realize what it is to be baptized, and by that fact you also realize that you cannot keep living like everybody else, you have a commitment to live up to, a call to live up to - just read the Sermon on the Mount (Chapter 5-7 of Matthew) - even though at times it might feel like too much.
Now, some things God asks us to do are right in themselves, and not to do them would always be wrong - like forgiving. It is right to forgive, wrong not to, and that applies to everyone.
Other things God would like us to do, but not to do them would not in itself be something wrong - for example, being a priest. It is not wrong not to be a priest - if it were, very few men could be saved, and no women at all! It is only wrong for someone who is called to be a priest not to become one. But it is a different kind of wrong than the 'wrongness' of not forgiving. I hope you can follow what I am saying.
The outcome is this: a man who does not forgive has to reverse course if he wants God to forgive him, but if a man who is called to be a priest says no, and gets married, he has disappointed God because he has said no to him. However, if he is sorry, asks God for forgiveness, God will surely do so, not withhold his blessings, and offer him all the means to live a holy and fruitful life as a married man. He does not have to get unmarried and become a priest for God to forgive and help him. Got it?
When we perceive a vocation we may feel 'forced' but it is not the overpowering force of a God who says 'if you don't do what I say, I'll condemn you for it'. It is, rather, the force of love. If God has chosen us out of many to bring us closer to him in a special way, making us servants of his people to take away their sins and make Jesus present in a real way in their lives through the Eucharist, it is a great privilege and dignity, but also a great responsibility, for God has placed much hope and many lives in our hands.
When you look at it with eyes of faith, there is really only one answer to give, no matter what the cost. When you look at it with eyes of hope, there is only one answer, despite our weakness. When you look on it with eyes of love, there is only one answer, despite everything else that attracts us.
The greatest gift God gave us is the ability to love. Freedom is a gift that God gave us so we could love. Understanding and faith are gifts he gave us so we could spend our love on something worthy of it, something that will take us beyond this life and make us happy for all eternity. Reason and faith free us from appearances and our limitations so that we can give ourselves fully to God, the greatest good there is, and in that way use the gift of these years of life he has given us here on earth in the most fruitful and beautiful way possible - doing what he would like us to do.
God bless. | 
