Sometimes the hit die look jarring. It's not like we see the fighter orc's hit points and DR. Are we going to complain when someone lands a 20 and-- as a result-- pull off bullshit?
@azhuresky vital spot detection isn't a separate skill. It comes from how the rogue skill 'sneak attack' is interpreted.
It depends upon which write-up you read, but some will mention that the reason sneak attack is effective is that the rogue is hitting a 'vital spot'. By derivation, that means the rogue has the skill to detect such a vital spot.
If Charlotte skin on her left side was still smooth and beautiful after she absorb the spike chain attack, then, as expected of bikini armor quality and performance....
@Fushiginiku True munchkin logic, I applaud you. And then we could derive that he could simply tell his non-rogue teammates where that vital spot is, and thus all hits on that spot from anyone count as backstabs....