bathtub vs vessel

bathtub and vessel both are nouns.

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

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

bathtub: a relatively large open container that you fill with water and use to wash the body
vessel: an object used as a container (especially for liquids)

bathtub (noun)	vessel (noun)
a relatively large open container that you fill with water and use to wash the body	a craft designed for water transportation
	an object used as a container (especially for liquids)
	a tube in which a body fluid circulates