cassock vs vestment

cassock and vestment both are nouns.

Word	Noun	Adjective	Verb	Adverb
cassock	Yes	No	No	No
vestment	Yes	No	No	No

As nouns, vestment is a hypernym of cassock; that is, vestment is a word with a broader meaning than cassock:

cassock: a black garment reaching down to the ankles; worn by priests or choristers
vestment: gown (especially ceremonial garments) worn by the clergy

cassock (noun)	vestment (noun)
a black garment reaching down to the ankles; worn by priests or choristers	gown (especially ceremonial garments) worn by the clergy