A lord's son
Junzi meant a lord's son. Not a good man, not a wise one: a man whose father was somebody, which in that world was the only qualification there was.
Confucius did not invent a second meaning. Noblemen had been fitting one for two generations before he was born, and the earliest clear case is from 615 BC. They used the word for a nobleman who behaved well rather than one who merely was one. That raises the standard without letting anybody else in.
Pointing it downwards
What none of them ever did was point the word downwards. In the surviving record of that century, not one speaker calls a man of Confucius's own station a gentleman. He did. The men he said it to were sitting in his own house, and hardly one of them owned a vessel.
Not a vessel
A noble family kept a set of bronze vessels and fed its dead out of them in the family temple. The vessels proved that a man had ancestors worth feeding, and the rank to feed them. Confucius was one grade below that doorway.
So when he said A gentleman is not a vessel, both words were in the room. The gentleman is the lord's son. The vessel is the bronze that proves the rank. He took the two things that certified a man, the birth and the bronze, and said that neither certified anything.
How far down?
Not as far as the legend says. He extended the word to the shi, the lowest stratum of the nobility, the one he came from himself, and kept it distinct from the common people. A student who asked to be taught farming was called a small man once he had left the room. He detached worth from ancestry inside a hierarchy he never questioned. That is still a large thing.