vi vs 6

vi is not a noun while 6 is a noun.

Word	Noun	Adjective	Verb	Adverb
vi	No	Yes	No	No
6	Yes	Yes	No	No

As adjectives, vi and 6 are synonyms defined as:

Other synonyms of vi include half-dozen, half dozen, six.

vi (noun)	6 (noun)
	the cardinal number that is the sum of five and one

vi (adjective)	6 (adjective)
denoting a quantity consisting of six items or units	denoting a quantity consisting of six items or units