Got to quote this as this is fundamentally incorrect at the levels we're discussing. Tank DPS outside of Alexander Savage is basically an optional luxury, yes. Good tanks will strive to maximise it for sure but you can get away with full vit turtle tanks in any encounter without a problem.
The issue with tank dps comes where an encounter comes along that says "your group needs a total of 7,000 dps or you hit the enrage and wipe". Most of this burden is on the DPS, but what people were finding is that when all four DPS did the absolute best dps they could possibly do in the gear they had, with max consumables and being all really amazing players... they were still short. So healers and tanks have to fill in the gap. If four DPS can do 1300 dps each and you need 7000 total to kill something, that extra 1800 would have to come from tanks/healers.
Suddenly the difference between a tank that pushes out 450 DPS and a tank that pushes out 900 DPS is literally the make or break factor to a fight. And this -is- what groups have been experiencing in Alexander Savage before they started amassing more and more i200+ gear.
Yes, the DPS have to meet the DPS checks first, but a lot of top end encounters are set up so it's simply NOT POSSIBLE to kill the fight unless tanks also contribute a certain higher-than-average amount of DPS. In such a case, yes, wearing one too many Vitality rings could be the difference between kill or wipe.