vault of heaven vs zenith

vault of heaven and zenith both are nouns.

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

vault of heaven (noun)	zenith (noun)
the apparent surface of the imaginary sphere on which celestial bodies appear to be projected	the point above the observer that is directly opposite the nadir on the imaginary sphere against which celestial bodies appear to be projected