a capella singing vs vocalizing

a capella singing and vocalizing both are nouns.

Word	Noun	Adjective	Verb	Adverb
a capella singing	Yes	No	No	No
vocalizing	Yes	No	No	No

As nouns, vocalizing is a hypernym of a capella singing; that is, vocalizing is a word with a broader meaning than a capella singing:

Other hypernyms of a capella singing include singing.

a capella singing (noun)	vocalizing (noun)
singing without instrumental accompaniment	the act of singing vocal music