This is shamelessly stolen from KI and is one of the best laid out explanations why +Vit/Def is not useful against HNM.
Eaglestrike said:Actually mobs aren't way above you in STR/Attack. If they were, you WOULD see better returns on VIT/defense indefinately. The problem is that mobs have a really high WDMG, Wyrms and such have like 200-250 WDMG (weapon damage for those who didn't get the acronym). Also, the reason melee, even DRK's with 100ish WDMG can still hit 0's on NM's is because level plays a play on pDIF...but only one way. If the mob is above you in level, it's a 5% cut to pDIF ratio against the attacker. So for those level 90 NM's you get -.75 to your attack vs defense ratio as a melee. As a tanking PLD, you don't receive anything close to that bonus and can never reach such a ratio because I think pDIF is capped at a certain ratio and only the level difference can modify it below a certain number.
That's why no matter your VIT/defense big NM's will still hit you for around 200 damage a hit, usually a bit less because your defense is > their attack, but your greatest damage mitigation is shield because it's straight -dmg taken and that does so much more than defense ever will for a PC