largeness vs vastness

largeness and vastness both are nouns.

Word	Noun	Adjective	Verb	Adverb
largeness	Yes	No	No	No
vastness	Yes	No	No	No

As nouns, vastness is a hyponym of largeness; that is, vastness is a word with a more specific, narrower meaning than largeness:

largeness (noun)	vastness (noun)
the property of having a relatively great size	unusual largeness in size or extent or number
large or extensive in breadth or importance or comprehensiveness
the quality of being pretentious (behaving or speaking in such a manner as to create a false appearance of great importance or worth)
the capacity to understand a broad range of topics