tanks not using cooldowns?

[Adire]

In 4 mans, using vengeance at 98% HP while I'm getting healed and using vengeance at 10% HP while the healer is casting stone are WORLDS different.

If you're saving Vengeance for when you're at 10% HP, you've waited far too long to even use it effectively. If your healer is in the process of casting a DPS spell and you pop Vengeance at 10% in the middle of a large pull, you're going to die most likely unless they have an instant cast ready to go immediately after, and you still may die.

If you had popped it sooner, it would have slowed damage down enough to actually give the healer more time to react and more time to DPS at the same time.

10% with a still DPSing healer is far more into Holmgang territory than Vengeance territory. You seeming to not understand this says a lot about your understanding of how cooldowns should work.

Vengeance and damage reduction skills like it and such are not for emergencies for healer mistakes, they're for preparing for high incoming damage or lessening damage to make more room for healer DPS. They're a proactive defense much like Galvanize is for SCH, not a reactive emergency button like Lustrate or Benediction are for healers and how Holmgang, Living Dead, and Hallowed Ground would be for tanks in regards to trash pulls (though given, Hallowed Ground technically can be used as a means to allow for more healer DPS)

If you use skills like Vengeance in this way in this situation you described, you are using them very ineffectively and pretty much guarantee your death at worst, and inefficiency at best. It's too late for them to be of much benefit since the next wave of damage will still kill you without assistance, and the healer will still have to spam heal you all the way back up and kill Vengeance time instead of being able to continue DPSing more with all the damage mitigated by a full length Vengeance.