I believe the answer is:
This could be a double definition.
'privateer's' is the first definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'vessel?' is the second definition.
(vat is a kind of vessel)
Can you help me to learn more?
(Other definitions for vat that I've seen before include "Large tank; purchase tax" , "Liquid storage receptacle" , "Tank for holding liquids" , "Large container for liquid" , "Large tank for liquids, or kind of tax" .)