vital vs life-sustaining

vital and life-sustaining both are adjectives.

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

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

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

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