There's formulae for how much damage you take and Defense just factors into that.

You should wear the Bronze armor though. The tank armor offers significant and noticeable damage reduction. The only reason the i17 armor has more VIT is simply because it's a higher item level. It is serviceable but you will likely get better performance in parties by gearing for your class.

Tank gear will typically be GLA, PLD, MRD, and WAR only and will have a lot better defense than other armor with the same item level. Also, you will usually find Parry on them but not always.

There isn't going to be any situation where you will have to choose between Defense and Vit if the items in question are the same item level.