vocal vs piece of music

vocal and piece of music both are nouns.

Word	Noun	Adjective	Verb	Adverb
vocal	Yes	Yes	No	No
piece of music	Yes	No	No	No

As nouns, piece of music is a hypernym of vocal; that is, piece of music is a word with a broader meaning than vocal:

Other hypernyms of vocal include composition, musical composition, opus, piece.

vocal (noun)	piece of music (noun)
music intended to be performed by one or more singers, usually with instrumental accompaniment	a musical work that has been created
a short musical composition with words

vocal (adjective)	piece of music (adjective)
having or using the power to produce speech or sound
relating to or designed for or using the singing voice
given to expressing yourself freely or insistently
full of the sound of voices