vocal music vs song

vocal music and song both are nouns.

Word	Noun	Adjective	Verb	Adverb
vocal music	Yes	No	No	No
song	Yes	No	No	No

As nouns, song is a hyponym of vocal music; that is, song is a word with a more specific, narrower meaning than vocal music:

vocal music: music that is vocalized (as contrasted with instrumental music)
song: the act of singing

Other hyponyms of vocal music include singing, vocalizing, strain.

vocal music (noun)	song (noun)
music that is vocalized (as contrasted with instrumental music)	the act of singing
music intended to be performed by one or more singers, usually with instrumental accompaniment	a short musical composition with words
	the characteristic sound produced by a bird
	a distinctive or characteristic sound
	a very small sum