My two cents about this type of armor:
In medieval times, the gambeson (the other name of the vapn) was the most cost effective type of armor available. Heavy linen and wool clothing provides a very important thing: PADDING
Without any sort of padding, metal armor (either ringmail or plate) is totally useless. First off, the metal plates would cut themselves into your body, second, any sort of blunt impact force would be easily transferred from the plate to the body. A well-made gambeson is actually highly resistant to cuts as well, contrary to popular belief.
The single drawback of gambesons is that they're thick as shit so they're very uncomfortable to wear for longer periods in warm climates.
I have worn metal helmets without any sort of padding before, and BOY... A slight knock on the head, and it hurts more than if i haven't worn any head protection (plus it rings like a bell on your head).
So here are some ideas off the top of my head to bring this item back as an actually viable alternative:
-Just allow gilding. I don't think shirts would become obsolete, as crafters would still use shirts.
-Increase its armor by A LOT! A well-gilded shirt can give over 100 UA, plus many other bonuses. It's fairly hard to compete with that, even if the armor is buffed, so i'd say the armor value that a vapn gives should be around steel plate level.
-Give it a unique buff to increase the effectiveness of equipped armor items by a percentage. This would make the vapn to be best in slot for fighters, but still useless to crafters with zero armor. This is mostly just for historical accuracy, not sure if i'm a fan of it.
Now of course if any of these modifications happen, the crafting costs should increase slightly in my opinion, since these modifications would make the vapn a "big boy" armor piece.
https://www.youtube.com/watch?v=ODS7ksbBRuE