viii vs eight

viii is not a noun while eight is a noun.

Word	Noun	Adjective	Verb	Adverb
viii	No	Yes	No	No
eight	Yes	Yes	No	No

As adjectives, viii and eight are synonyms defined as:

Other synonyms of viii include 8.

viii (noun)	eight (noun)
	one of four playing cards in a deck with eight pips on the face
	the cardinal number that is the sum of seven and one

viii (adjective)	eight (adjective)
being one more than seven	being one more than seven