Still I would think mind would help more with an under geared tank than vitality would. You will be able to help him stay alive that much longer to absorb damage for you and you will be able to heal your dps better that might end up pulling aggro. As for yourself tanking I'm not sure but again I'd say mind just because you will ideally be able to replace hp faster than you will be losing it where as the larger pool of vit will really only help when it comes to bigger hits.