vat vs vessel

vat and vessel both are nouns.

Word	Noun	Adjective	Verb	Adverb
vat	Yes	No	No	No
vessel	Yes	No	No	No

As nouns, vessel is a hypernym of vat; that is, vessel is a word with a broader meaning than vat:

vat (noun)	vessel (noun)
a large open vessel for holding or storing liquids	a craft designed for water transportation
	an object used as a container (especially for liquids)
	a tube in which a body fluid circulates