No, there is no sin here, especially since living out your vocation doesn't manifest itself in a single moment but over the course of an entire lifetime. A vocation is a call -- a call God grants in order to maximize our happiness, but He doesn't condemn us for making mistakes, not hearing His voice, etc., insofar as the decision here is not evil or inherently in contradiction to one of the Lord's commandments, etc.
Ok, from what I read on the discernment book “To save a thousand souls” (for discerning diocesan priesthood) there are three categories of vocation (pardon my approximate language, I am trying to recall this the best I can):
the universal vocation in the hearts of all men (to love God above all for His own sake, and our neighbor because of Him)
the primary vocation ( consecrated life, married life, generous single life)
the secondary vocation (how we serve the Lord, including the talents we have received)
Clearly discernment is a process. It takes time. It needs guidance - especially from a spiritual director. But it is fairly possible that one may after a significant time of prayer and discernment (meaning, usually, years) determine that God’s will is neither for him to be married, nor for him to become a priest or a religious: God calls him to live a generous single life.
However, I may be mistaken here but I think you are at an early stage of discernment, because you speak of “choosing” a vocation rather than of “receiving” a vocation. We do not choose or figure out our vocation, we receive it from Christ. It is neither what we want to do, nor what we can do: it is what the Lord would want us to do.
As for whether it would be sinful to not follow the vocation to which God calls us: God above all else calls us to holiness. To resist this universal vocation would be sinful indeed.
But God also, in His wisdom and loving kindness, endows each of us with specific gifts and talents, and calls each of us to a specific state of life in which we will do the most good. This may not be what we expect. Christ was able to teach the doctors of the law at age 12, yet His vocation was to work as a carpenter’s aid for most of His life. In doing so, He gave glory to the Father and saved souls, because He was doing God’s will and responding to His vocation.
What if a soul decides that she will not follow God’s calling? Well, our primary vocation is not a command, but a suggestion. A recommendation. God knows that this is the most fulfilling life we can live. That is how He wishes us to serve Him and help souls. If we resist it or chose another path, God will still bless us with His loving grace, but we will never attain all we would have been able to attain had we responded generously to His call, whatever it may have been.
His call at this point may be as simple as to enter the process of discernment and talk to a spiritual director. The Lord does not ask much from us, just for us to trust Him and to agree to take the one step He asks of us today. If I am discerning matrimony, I am not expected to know that I will marry the woman I am starting to date. If I am discerning priesthood, I am not expected to know that I will be a priest as I enter the seminary. You see my point?
I hope then that you will deepen your discernment by an increase in quiet prayer time (even a few minutes daily), by bringing this matter possibly once a week to the Lord in the Blessed Sacrament, and by discussing the matter with your confessor for spiritual direction. One step at a time, after all what could be more fulfilling than to realize that God is calling you specifically to do something and to respond to that call?