| Kephra |
I'm fairly certain that in the event DR completely negates the base damage of an attack, all other effects of that attack (with the specific exceptions of touch attacks, elemental damage, and level drain listed in the SRD) are negated. On the other hand, I've seen it ruled both ways for sneak attack, and even the official FAQ/rulings/sage-advice have flip flopped on it.
For me, that's the key. Like elemental damage.
Sneak attack damage isn't multiplied by crits (unlike enhancement +damage, or stat +damage), it's added at the end.That's the same way elemental damage is handled.
As far as 'how do they DO that damage if they can't get through the DR to get at the vitals?' goes, it's got easy examples if you're in the mood.
Amazingly hard scales, yeah. Pull off a sneak attack, and you got through the seam between two scales as the beast turned (the additional damage was enough to penetrate DR while the base was insufficient... the system doesn't allow for hit locations, particularly golden-bb type hit locations, but the narrative could describe the successful sneak attack in these terms).
(Apologies to Tolkein, but it's an easy one many people might recognize) Bard the Archer vs. Smaugh the Dragon: Critical hit with sneak attack? Arrow of slaying? Or outstandingly called shot? Depending on your combat narrative, any of these could be the case.