There are several different meanings of “to know” being used here. One is the more colloquial, “I know I’m going to” which implies that there is such a degree of certainty that no time needs to be spent contemplating its alternative. The other is the philosophical “to know”, which is more mathematical, and implies 100% certainty. Since there are many factors you don’t “know” – you might get run over by a car, ending all your vocational plans – you cannot know in the philosophical sense until the event has actually come to pass. In this latter sense, what the priest said is true.
Also, I believe that God may have various intermediary and temporary callings in our life, in order to lead us in the desired direction. I could thus say, "I know I am being called to marriage right now" but I cannot say “I know that I am going to *be married.” *I could discern a call to marriage, get engaged, and, returning to my morbid example, my fiancee could get run over by a car and God could then reveal that now he wants me to call me towards the priesthood.
“Towards” is the word I like to use. It reflects my own opinion on vocations. It seems (to me) that God calls us towards particular states in life, but it may not be a calling all the way to that destination. I do not need to worry about the final destination; my only concern is "What is God calling me to do right now?"