Yall gona make me record a coin counter fight >< ....
Not sure why, But DEF and VIT seem to have a relationship with one another as far as effectiveness gose. Not sure why, but , Def dose deflect more normal damage then vit, however if coupled at a (roughly) 2/1 (def/vit) ratio your def gose much further and its not simply because your adding more def..
for example , start with 400 def and 100 vit, slowly start adding vit stats, the closer that number gets to a 2/1 ratio the higher damage is deflected per vit added. Its as if the vit , for lack of better terminoligy, "unlocks" the defensive potential. The main reason I Strongly discourage HP builds over def and vit is simply due to the devistational effect of rapidly reocurring strong physical attacks. If this test is done using the same formula but with in reverse order 2vit/1def the outcome will be more damage taken, DEF is stronger then vit, but you do need the one to realize the others full potential.
Hp dose help with magical based attack, and I would agree that as far as magic defenses go, the current tank build is broken. That being said, there is currently no boss fight in game that deals more magic damage then physical damage. High HP will help you survive possably two to the massive hits, be it magical or physical, but Many times I have seen tanks go down simply because cast times on spells/buffs just could not keep up with devistational boss TP moves.
With High DEF and VIT my healer/s only need to focus on ss untill a magical hit takes some hp off then the ocasional cure, but I have been told many times, I take far less damage and I have parses to prove it. botom line, VIT/DEF > HP the goal is'nt simply to survive attack but to make your WHMs job easier , The easier you make it, the fewer heals you will need and in turn the more dps can be put out. A well built turtle tank using this philosiphy can prove to put more DPS in the fight then a well build DD/TANK