welkin vs vault of heaven

welkin and vault of heaven both are nouns.

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

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

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

Other synonyms of welkin include celestial sphere, empyrean, firmament, heavens, sphere.

welkin (noun)	vault of heaven (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