Vanity slot restriction concerns.....

[RyuujinZERO]

I'm not sure Op realises that "shit flows downhill" in FFXIV.

There is little to no "Disciple of Magic" seperation in gear, generally caster gear has the "All classes" descriptor - so any class can wear caster potato sacks, be they tank, gatherer or archer. Conversely, even cloth martial gear (ie. cotton actons), are given the "disciple of war" descriptor, and cannot be worn by casters, crafters or gatherers. But can be worn by tanks or martial DPS classes.

The full list of who can wear what stands as this:
Tanks: All plate armours, scale armours, leather armours, martial cloth and caster cloth
Dragoons: Scale armours, leather armours, martial cloth, caster cloth
Archers, Monks: Leather armours, martial cloth, caster cloth
All casters: Caster cloth only

This downhill flow means that technically ALL gear is "tank gear", while "caster gear" means "The small number of items designated as 'all classes'". If SE does as you think, and limits vanity options to the gear a class can normally equip, then casters get screwed - not even being able to wear a cotton shirt or kecks; while tanks can legitly wear a frilly maid dress into combat (No joke, they can already do that just their stats would suck)

It only seems fair that casters have the ability to display heavier armours as their vanity options - there's pretty good grounds to allow this too; Louisoix wore/wears an acton and plate greaves (disciple of war only), Paplyamo wears plate gauntlets and leggings (tanks only) - plenty of NPC's with known classes flagarently violate the established rules of what classes can wear what armours in order to look cool. And speaking as a caster myself, I'll be extremely sad if I'm forced to only wear variations on a potato sack, while my tank colleagues can tank wearring whatever they damn well please

Fundamentally, I don't see any good reasons for restricting cosmetic options