That is correct. Vessel maker is a secondary craftsman with rules further down (the parentheses are mine):
"It is allowed to build a secondary craftsman (e.g. vessel maker) such that he uses the same resource (e.g. clay) as the primary craftsman he depends on (e.g. potter)."
The rules even state that "in some circumstances, this may mean that it is impossible to use either of the craftsmen for upgrading a monument".
For a newb to the game, what gameplay advantage does this create.
Is it a blocking move?
Is it to steal the use of the resource from another player depending on turn order?