yea, personally I'm not a big fan of the current implimentation of the vault. I'd like a "free" option where you can open it at say 50 cards but it's like 39 common, 10 rares and 1 epic, or you can upgrade it for let's say 50 diamonds and then it locks again and needs to be filled with another 50 cards or something. Do this 5 times to get the max rate and we are good. Diamond users are happy because they get a great deal, freemium users are happy because it's not a feature thrown in their face every crate that they don't get.
Also throw in some credits, it's a vault after all, not a card binder =p