vault of heaven vs firmament

vault of heaven and firmament both are nouns.

Word	Noun	Adjective	Verb	Adverb
vault of heaven	Yes	No	No	No
firmament	Yes	No	No	No

As nouns, vault of heaven and firmament are synonyms defined as:

vault of heaven and firmament: the apparent surface of the imaginary sphere on which celestial bodies appear to be projected

Other synonyms of vault of heaven include celestial sphere, empyrean, heavens, sphere, welkin.

vault of heaven (noun)	firmament (noun)
the apparent surface of the imaginary sphere on which celestial bodies appear to be projected	the apparent surface of the imaginary sphere on which celestial bodies appear to be projected