life-sustaining vs vital

life-sustaining and vital both are adjectives.

Word	Noun	Adjective	Verb	Adverb
life-sustaining	No	Yes	No	No
vital	No	Yes	No	No

As adjectives, life-sustaining and vital are synonyms defined as:

life-sustaining and vital: performing an essential function in the living body

life-sustaining (adjective)	vital (adjective)
performing an essential function in the living body	manifesting or characteristic of life
	full of spirit; full of life
	performing an essential function in the living body
	urgently needed; absolutely necessary