Vitality
Typically used with: Endurance.
In short: tough it out. The Vitality skill is used when a character is performing strenuous activity or hanging onto life by a thread. Many environmental effects require a Vitality roll to shrug off, such as extreme heat, cold, thin air, or no air. Many spells ask for the Grunt Save which involves Vitality and Endurance. A character heals an amount of HP equal to their Endurance + Vitality for each period of rest.
Difficulty
The DL for environmental effects is extremely varied. In general, it should be based on the extremeness of the climate. Your GM has the final call, as always, for determining DL for these.
If a character that is suffering from environmental effects (heat, cold, etc.) fails a check, he or she moves down one step on the Knockout Track. Each hour a character is in a harsh environ, a new roll must be made.
Opposed Rolls
Vitality is sometimes used in conjunction with Endurance against a Spell Attack or Special Power.