Some very extreme examples here and outliers. The tank should never be at 30% health if the healer is doing their job. All of the damage in this game can be accounted for before it's ever taken. You shouldn't be waiting for the hit and then starting the cast. Unless you are experiencing huge fluctuations in pull sizes, the rate of incoming damage is relatively the same every single time. The biggest incoming damage rate decreases come from cool downs which EVERY tank should be using and rotating thru unless they are being saved for a specific instance. Vit accessories have zero net effect on this what so ever because if you get to the point where you "NEED" the extra hp then you've already made an error. The savage meta is around 16k and out of savage everything can be tanked comfortably with 14k ish hp and with the gear upgrades lately you should be able to hit the latter with 190/200 left side alone.

That being said good healers, like tanks, are the exception rather than the rule. I personally run with pentamelds but different situations necessitate different setups. There is no situation that necessitates a full vit build. But i will concede that the chances of getting a garbage healer/tank in df is substantially high and during leveling (at level) a 2/3 vit/str or str/vit setup can be beneficial in some cases while leveling especially if you are still learning the class.