song vs vocal music

song and vocal music both are nouns.

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

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

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

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