Vocation is a bi-polar system. It is a connection between the One who calls and the one who responds.
But the fact is that it is a commitment:
God keeps repeating: “I am with you, till the end of time”. It is commitment, permanent.
Man responds: I shall walk with You, that too is commitment, hopefully long lasting.
Vocation then is an attempt to place side by side and one on top of another two commitments.
That of the robust Godly Foundation and that of a fragile human attempt.
But as “everything lives and moves in Him” even these commitments live, thanks to HIS eternal presence.