General Importance:
* Essential
* Crucial
* Necessary
* Indispensable
* Critical
* Imperative
* Fundamental
* Key
* Paramount
* Primary
* Significant
* Major
Life-Sustaining:
* Life-giving
* Life-supporting
* Essential for life
* Necessary for survival
Energetic and Lively:
* Animated
* Vigorous
* Lively
* Full of life
Specific to a situation:
* In a medical context: Life-threatening, critical, urgent
* In a business context: Strategic, pivotal, core
* In a social context: Crucial, decisive, defining
The best synonym for "vital" will depend on the specific situation and the intended meaning.