Vitality can be an effect printed on the card or given to a unit by another effect. The status is recognisable by a green banner in the top-left-hand corner of the unit. The number inside the banner indicates how many more procs the status has before it disappears.
It is important to note that Vitality and Bleeding are mutually exclusive statuses that cancel each other out. If a unit with 4 Vitality is given 2 Bleeding, it will be left with 2 Vitality. Similarly if a unit has 2 Bleeding and is given 6 Vitality, it will be left with 4 Vitality.