Things have been rough on Mido lately, especially with the encroachment of urban areas into places that were traditionally inhabited by tree-dwelling folk. This has created a more pressing need for what basically amounts to forest rangers, of which Vin is one. Her detail keeps her close to her village, where as she acts as a guard to ward intruders away, shoot them if they defy her warnings, or alert the whole village in the case of a larger incursion.
It should be noted that the forest-dwelling Midoans typically take a dim view of the use of firearms, as they are seen as a tool of the enemies of the woods. Vin took an interest in them from a young age, however, and continues to use them despite her elders' distaste for them.
Holy crap I hate drawing guns.