The problem with armour at the moment is the everyone is going to look the same, because we are all going to wear the current best raid armour. (Except one exotic) As new armour comes out, we may or may not like the look of it, but because of the light system we are forced to wear the one with more light. With this system we pretty much have no say in what we want our guardians to look like (except for one exotic piece). The vanity slot will only be cosmetic, you will only have the physical look of armour but not the stats, so vanity slot allows us to wear any kind of armour without any drawbacks, we can make our guardian look differant but still stay at max level. Also we should be able to wear multiple exotics in the vanity slots, since it's only for cosmetic looks it shouldn't give anyone an advantage. If in PVP you want players to be able to see what the enemy is wearing without inspecting them then simply disable vanity in PVP.