Maybe our true purpose in life is unknowable? I agree that purpose and vocation may be two separate things.

For example, someone came into my life briefly and actually changed my life. And he never even knew it, didn't have a clue as to the huge effect he had on me. And never will. (Don't mean to be mysterious, it's just a long story, too long for here.) I'm not saying that was his true purpose in life, but he unknowingly changed my life.

Another example, but fictitious - A guy is in a bank when a robbery begins, and is killed by a bullet that would have killed a pregnant woman if he hadn't happened to accidentally be standing where he was. In time, the woman has her baby who grows up and finds a cure for cancer. Maybe that man's true purpose in life was to be standing in that place at that time and save the life of that child.

Interesting post, de meliora - thanks for making me think.