This is way too OP unless you're talking about giving something like this to ALL of the tanks, which would be awesome, and solve a lot of the STR vs. VIT crap that permeates this game's tanking meta's mentality. I would absolutely love it if all the tanks got an ability that did this, and could keep their tanking and dps stances/buffs along the side. Then you could simply stack VIT like logic would suggest that you should, and toggle the "tank cleric" stance as appropriate. This would also make things like tanking outside of tank stance far more viable as you could keep the VIT stance on and switch to Sword/Defiance/Grit-off, or keep the tanking stance on but toggle to your STR stance.

TLDR awesome idea, but should be an option for all the tanks. Giving it to just one would make that one crazy-overpowered.