Vim is the resource used by the Defiler metaclass (Reavers, Corruptors, Doombringers, and Demonologists). Vim does not regenerate but is instead regained in several ways, all involving combat. Fatigue plays no role in determining vim costs.
If you don't have enough Vim to cast a talent, you can pay with twice as much Life instead.
The vim pool grows by 4 points every level and is not influenced by any stats.
Ways of regaining vim: