In any case, vengeance is becoming a very important component of dps, I often do world bosses on my blood dk, 534 item level, full dps gear. For example, on galleon, I got tired of trying as frost because (unless aoeing adds which I never tried) I could never keep up with frost mages of my same item level, now I just go blood, full dps gear, tank it, 120k vengeance, 206k dps, second dps has 150k or so. I don't even take adds, could do 250-300k with them.
Blood dks are also scaling BAD with vengeance compared to other tank classes, they are the highest dps tank spec pre-vengeance if I'm right, but with vengeance, prot paladins, guardian druids and even brewmaster monks have just more survival which allows them to pull even more dps than blood dks could. Prot warriors have some limits to this due to playing with second wind, which forces them under 35% hp, but they still scale better with vengeance.
I know old content is old, but prot paladins are soloing warlord zon'ozz eating all the stacks of focused anger from start to end doing over 200k dps which a blood dk can't even do by bouncing the ball and increasing the damage taken by the boss, guardian druids are pulling all yor'sahj trash and aoeing the trash + boss down with 845k dps without even dying.
So for some tank classes, not only the damage becomes INSANE when standing in fire\pulling all, but they also have special abilities to soak them and even IMPROVE their survival.
A prot paladin soloed galleon, which even if intended for 463 item level players or so it's still a 40-man boss and hits 220-320k on a tank who isn't using cds, they have so insane vengeance from melee hits + adds that they can do the 320 millions damage in less than 15 mins while still aoeing adds.
A prot paladin soloed primordius normal in 537 item level, on asian servers, the link isn't easy to find, but this is caused once again by the vengeance which also translates in more survival.
The change coming in patch 5.4 will limit this a bit, if a tank has 800k hp, he'll have a vengeance cap of 266k in 10-man raids (was it 33% of total hp?) and 400k (was it 50% of total hp?) in 25-man, but still quite high.
Personally as a blood dk dps starts skyrocketing from 80k vengeance to higher, so this won't affect the vengeance we can reach in tanking current raids\soloing cata stuff.