system
Given the following post, respond as if you're commenting on Reddit:
human
[asoiaf] How, in the domesticated Seven Kingdoms, is it possible for the Hill Tribes to exist in the Vale, plundering without being hunted down by the Knights of the Vale or by other official authorities?

They're almost like the Wildlings but south of the Wall.
gpt
The Hill Tribes are broken, fractured, impoverished, using barely any armor or weapons of quality but what they can -- rarely -- steal. They're uneducated, lack tactics, organization, leadership, and are compared to the northern Wildlings yet unsophisticated, untrained, and frankly rather stupid. Tyrion and Bronn and Tywin played them like children. The Vale knights and cavalry are amongst the most powerful and dangerous and martially skilled forces in all of the world. 

The hill tribes are the rednecks of the Vale and kept in firm check. Why the Arryns haven't simply wiped them out is probably down to simply not wanting to slaughter families whole. If this was the Westerlands, yeah, the Lannisters would've erased them by now. The tribes probably aren't enough of a problem to rise to that level.