Vault Card Questions

I read the basics on vault cards but I was wondering how do the daily challengers work between characters ? because I swear I seen some that share progress to a challenge and others have been separate or able to do on multiple characters. Note I said daily not weekly.

I assume you can buy the equipment more than once ?

Is there any reason to not use Vault Card keys on a high mayhem for better loot ?

It seems higher level characters progress more towards vault card XP, both with normal XP gains and completing daily challenges, is this true ?

Is it better to buy items with a low key cost first ? because you get a vault key if you open a duplicate from before and chests can contain higher key cost cosmetics or does it matter ?

There could be reasons - e.g. I redeemed on Gu4rdian Ang3l shotgun at level 1 to give to my grenade-only Moze. Its main effect is level-independent, so it was handy for that.

This is buggy as hell and you can come across all possibilities you mentioned.

Also some daily challenges once completed stay that way when they come again.Gearbox fixed some of those but that still happens.

You can redeem your Vault Keys to get another of the four weapons & items featured on the card after unlocking them.

Each level you reach will reward you with a chest, and from the chest you can get Keys, Eridium, or one of the Items on the card unlocked in addition to featured loot from Maurice’s Black Market if I recall correctly.

Depending on how quickly you level up the card and accumulate chests, it might be worthwhile to save your keys and see if you can get Card rewards unlocked via chests. That way you save your keys for duplicates of the usable gear, as some of it is incredibly useful.