Often times larger than necessary health pools just become MP dumps for healers. Health pools should never be looked at as the biggest is the best, for if that were the case the WAR would win hands down. Having sufficient health to absorb significant damage before requiring attention is the target. Now, what is sufficient health? That is really the question and one that cannot yet be answered for the factors are myriad.
Side note: Stoneskin is as of right now nearly useless as anything but a pre-pull buff. Cast time and MP cost are too high and efficacy too low to be of value during battle.
In what way am I wrong? The mana cost at level 50 is 3x's the cost of Cure (399 for Stoneskin, 133 for Cure) has a 3 second base cast time (50% more than Cure) and only shields at best 18% of PHYSICAL!!! damage. There is a decent write up of Stoneskins problems in the healer forums if you want to check it out.
I'll make up some fake numbers here since you're 50 and I'm not so idk what health pools are at that level. At 4000 HP Stoneskin will shield 720 HP (that is not much for 399 mana) but at 5000 HP we are sill only getting 900 HP of physical protection. That is not a huge jump in bonus protection for a theoretical 125 VIT. You also mentioned heals that are better when health is higher. I've looked over CNJ, WHM and SCH and see nothing of the sort. What are you talking about specifically? The SCH does get a shield of sorts, but is is based off its healed amount, not target HP.
Ultimately what that means is Stoneskin is not good for the tank during the fight since Cure heals all damage, not just physical, has a lower mana cost and is faster. It is also not going to be used on DPS since it has such a long cast time which prevents it from being used reactionary to immediate threats.
When would higher HP be a bad thing? When mitigation is sacrificed for it, as might be the case with STR. There is a balance that must be struck between mitigation and health and both suffer from diminishing returns. Having a huge health pool that gets drained quickly becomes, as I already said, an MP dump for healers since they will be required to spend more time casting heals on the tank. If these basic points have some how gotten past you, I fear for your ability to do this job effectively.
However, you might be absolutely correct in saying that VIT is more important than STR (for these 30 points; though if for these 30, than for any). The fact is we just don't know how effective STR is in block and parry mitigation. Perhaps it will be too insignificant to be worth investing too heavily in. My point is that all things must be considered and just blindly saying VIT is better because it gives you a bigger HP pool is foolish. Tanks must be able to balance enmity generation, mitigation and HP. Not one is more important than the other.
Enjoy!