It all boils down to 1 thing: effective health vs mitigation/avoidance(lol). You could also think of tanking as a Tripod (thanx yoshi for letting me use this idea lol). There is effective health, mitigation/avoidance, enmity. If you try to work on 1 thing, such as enmity, your mitigation and hp will suffer, if you try to work on mitigation your hp and enmity will suffer. There is also the problem of materia only socketing into specific pieces. Enmity and Vit overlap on the sword, Def n Vit overlap on the chest. While Vit has the potential to be a super stat, in that it increases mitigation of many things, slightly increases hp which increase enmity through Spirit's Within, it is still very broken and lack luster in that we need too much. Enmity does nothing outside of allowing us to build more threat so whm's can cure bomb more. However a pld comes with higher than average mitigation, this I believe true and will stand by it. That leaves only 2 other things a pld needs to work on, Enmity and HP and by stacking HP coupled with Spirit's Within, this solves that issue. HP shines for pld tanks, it is the go to stat for now until the day they nerf our mitigation (heaven forbid). But vit has potential.

If anything stat priority should go HP>Vit>blahblahblah(personal preference after). Simply because HP pools are so low in this game and it is very easy to pick a tank up from the dead, aka 200 hp left or less not actually dead, and top him/her off. Every time the tank is topped off, that extra hp is helping.

tl:dr pld comes with a ton of mitigation leaving hp the key stat, even for enmity with Spirit's Within