Vigilance quests were added in module 6: Elemental Evil. Vigilance quests are hourly quests; each NPC changes which quests they offer at the top of each hour. After turning in a Vigilance quest you may not retake that quest again during the same hour, and must wait until the NPC offers it again. You may only have 4 Vigilance quests, from any any zones, taken at a time. If you have 4 Vigilance quests, NPCs will not offer any further Vigilance quests until you complete or abandon one of the vigilance quests in your journal.
Vigilance quests are offered in the 4 elemental zones: Drowned Shore, Reclamation Rock, Fiery Pit, and Spinward Rise. Each zone has 3 areas, and you must complete 8 Vigilance quests in each area before being able to take Vigilance Quests in the next area of that zone.