What I'm hearing from people playing DRK at the moment is that Darkside is very very easy to keep up with no bonus piety, just using the few mana regen skills the class has. It seems it's only more complicated to keep up once you get Dark Arts.

It definitely seems like pure VIT is the way to go for DRK for progression tanking like all tanks. Once you get enough Vit to feel comfortable tanking though I guess we'll need more complicated theorycraft to work out whether flat STR or more PIE for greater Dark Arts use will equate to higher damage/threat. My gut feeling has me thinking that Dark Knights will be able to tank just fine with a base level Piety and STR/VIT will be their main stats, similar to how paladins and bards get by just fine without any... but Yoshi did mention needing PIE at some point in a live letter so I guess we'll see.