Vocation is about the mystery of sacrificial love. The question of vocation is the question of how God has made me to give myself away in imitation of His sacrificial love. I can give myself away for the one in marriage or for the many in priesthood or religious life. I have to choose. Some people might be equally capable of both, others maybe only for one. I think that is the case with me. Maybe I have just been a priest for so long that I can't imagine being a husband, but I really think God made me for priesthood and not for marriage. I know many fine men who are wonderful husbands but who, I don't think, would make good priests. (Warning -- the previous statements about "either/or-ness" of vocations are not the prevailing wisdom so I am likely wrong. Also, I am assuming priestly celibacy as the norm.)
How do I love best? That seems to be the way that God gives us the nudge in one direction or the other. Do I need to focus my love on one person so as to make it real? To experience its real demands that demand to be fulfilled? And which include the tenderness of unity and loss of self in the other? Or do I love the One in the many manifestations of His love and mercy? Does my heart want "to save a thousand souls"?
In either case, vocation is very much about mystery. The mystery of the other. The mystery of the self. The mystery of the Love, outside of both the other and the self, that unites and fulfills.