Catch up to what? Previous tier gear is catch up gear. If crafted weren't as powerful as it is now, it wouldn't be used at all outside of glamour. It's too expensive and provides too little benefit. For most people it's easier to wait for the current tier tomestone gear than to make the amount of money required to buy it or skill up your crafter/get all the mats so you can make it yourself. It's not a problem.
VIT on everything. The only real balance issue I see is a few very hardcore groups able to clear final coil a week or two after it was released. But it wasn't just because of tank accessories, they had everyone in crafted accessories so that healers and DPS had enough VIT to meet HP checks. With VIT on everything, you balance the raid damage against the higher amount and you would have the discrepancy between crafted and normal gear that's causing the imbalance.Regarding your solution, do you mean VIT on literally all accessories or just put both STR and VIT on tanking accessories. If it's the first, then we'd just need bosses to do more raidwide damage to compensate for a partywide increase in health. If it's the second, then they'd probably need to be class locked to Tanks so that MNKs and DRGs don't start using them. Either way, wouldn't it greatly invalidate the purpose of crafted accessories anyway?
If Crafted accessories were then restored to just being a normal last tier accessory with materia slots (akin to what belts are now), then they would still ususally be BiS for some classes, but they have to sacrifice some VIT to do it (like belts are currently). The difference would also be very small, so it would not be that big a deal.
You could even make the crafted have 3 full secondary stats and no primary stats. This would at least give the primary stat materia some purpose.
Tanks don't stack defensive stats because the current ones are of limited to no use at all. VIT doesn't mitigate damage, it just allows you to take bigger hits. Once you have enough VIT to take the biggest hits, there's no reason to stack it anymore which is why a lot of tanks stop bothering with it after a specific threshold. What would really be bad about tanks being forced to take VIT only accessories is they they would becomes VERY low priority upgrades since they'd be mostly pointless upgrades at that point. How fun is it to be forced down a pointless upgrade path? Of course you could also make tank only acc STR + VIT, which would be the only good way to do it if you were going to force job specific accessories.After all of this mucking with the meta, the only way most Tanks nowadays would willingly stack a defensive stat is if it were actually necessary to survive a given battle. Given the way that secondaries currently work right now(they seem deliberately designed to not make too much of a difference just as long as you aren't egregiously wasting them on Accuracy or something), something like that seems unlikely.
Parry is different because many tanks don't find it useful at all. They don't feel it changes healer behavior and parries will just lead to overheals (that's a separate debate).
If tanks had stats that actually made them take less damage in a somewhat reliable fashion, I imagine those would be more popular than VIT/Parry currently is.