Main thing killing tanks or having problems with bosses for tanks is the lack of hp to survive the big hits. 30 points in vit just does more no matter what since these are static.. you gear changes and with that you need for those "3 points in dex for next tier" was just voided.
All in all the big tank killer.. is huge magic damage. That makes vit for these pitiful 30 points far better. At end game you get more then 30 vit on a lot of armor.