I think "vessel" is where we went different directions.
To my thinking, every person is a vessel. I mean... some are empty, some are a quart low, some are full, and some overflowing. And perhaps they are not full of the same thing, but... they are all vessels nonetheless.
The cistern is of a similar typological metaphor-allegory-analogy:
Jeremiah 2:12-13
[12] Be astonished at this, O heavens, and be horribly afraid; be utterly desolate, says YHWH, [13] For My people have committed two evils: they have forsaken Me, the fountain of living waters, and they have quarried from-of themselves cisterns, broken cisterns that can hold no living water!
The "cavity" of the man is likened to a cistern, that is, the chest cavity, which contains the heart.