Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
vital [ˈvaɪtl] adj (essential, important, crucial) жи́зненно необходи́мый/необходи́м (full of life, person) живо́й, жизнеспосо́бный*жизнеспосо́бен (necessary for life, organ) жи́зенно ва́жный*ва́жен
of vital importance (to sb/sth) жи́зненно ва́жно (для кого́-н/чего́-н)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.