Learner's definition of VITAL
1 [more vital; most vital] : extremely important
These matters are vital to national defense.
He played a vital [=key] role in guiding the project.
The sciences are a vital part of the school curriculum.
It is vital that you follow all safety procedures. = It is of vital importance that you follow all safety procedures.
2 always used before a noun : needed by your body in order to keep living 3 [more vital; most vital] : very lively or energetic