When you get extra sneak damage damage from a weapon feat or stance (like knife or fencing), do you need to qualify for a sneak attack to use that extra damage (i.e. flanking the target). Or do you just get an extra die of damage that can get negated by anything that can normally not be affected by a sneak attack.
It's the former. As I always say, Sneak Attack Damage is a conditional type - in order for it to apply, the character has to be vulnerable/flanked, has to have vitals to strike, etc. etc.