Typically used with: Endurance.
In short: stay healthy. The Vitality skill is used when a character is fighting off disease, poisons, tranquilizers, and sickness. This skill essentially represents your ability to flex your immune system and physical composure. If you want to get on all of the carnival rides, even the spinning ones, Vitality is for you.
A character heals an amount of HP equal to the sum of their Endurance and Vitality for each full night of rest.
Every poison and disease a character might encounter has its own DL. The Life and Death chapter has some example diseases and poisons.
Vitality is also used in those unfortunate situations where your character needs to keep his lunch down or his pants stain-free. The circumstances for these challenges are as varied as their effects, so the DLs are variable.
Vitality is used against some Magic Spells and similar effects. In this case, Magic Defense can be added to the roll.