I'm guessing here but I like this sort of question.

Without the vapor barrier it must just pass through the bag into the air. To test this I've weighed my bag at night and in the morning and found no measureable weight change.

With the vapor barrier (if it doesn't collect on the vbl) then it must be on the person and/or their sleeping clothes or it must have escaped through the head (open) end of the sleeping bag (and liner).

From a practical matter I've had no trouble finding the moisture when I've experimented with vbls. It is all over me and the inside of the vbl. I've tried it twice and never made it through the night without getting so wet that I aborted the mission.