A patient's vital signs are their important body functions, such as pulse rate, that shows they are still alive. Use the adjective vital to describe something that is important and necessary, or a person full of energy.
Vital descends from Middle English, from Old French, from Latin vītālis, from vīta, "life." If you are vital to the organization you work for, it means they cannot live without you — or at least that you're an important part of the team. If you're a vital force in your church volunteer group, you probably are a leader with lots of energy to give.