zenith vs vault of heaven

zenith and vault of heaven both are nouns.

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

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