In recent years, Magic players have seen the power of being able to exile a card from the top of the library and be able to play it that turn. Even with the possibility that it will be a card that you can't cast and will lose "forever," the upside of essentially drawing a free card is very powerful. Oracle's Vault lets you do just that by paying 2 colorless mana and tapping it. But each time you do this, it gains a brick counter.
Once Oracle's Vault has amassed three brick counters, you're able to access its second tap ability. This activated tap ability doesn't require any mana to activate, and allows you to also exile a card to be able to play that turn. However, you don't have to pay that card's mana cost in order to play it. Not only is this great, but you don't have to remove any counters from the Vault to use this ability. Once there are three brick counters on the Vault, you can use this ability every chance that you get to tap it.
There are so many possibilities to break this card wide open in Commander. There are so many ways to untap artifacts, and this can get up to three brick counters in a hurry. You could play the majority of your deck in a turn in some artifact-happy decks utilizing the Oracle's Vault. But what about competitive 60-card Constructed play?
Is this card truly as good as it looks? It would seem that this card is on the verge of being at least Standard-playable, as 2 mana isn't a bad price to pay for that effect. It is 4 mana to cast, and doesn't have an immediate effect on the game, so it may be a bit too slow. But the raw power of this card is amazing.
How about Modern, where the Tron lands can create tons of colorless mana? Both Tron and Eldrazi decks may find a home for this card, but how does it fit into the current lists? Affinity probably doesn't want to pay 4 mana for this. It's just hard to say how this will work without other ways to throw brick counters onto this.
Being only a rare and also being a promo, it's not like Oracle's Vault is going to be hard to find. This is one of those cool "sleeper" cards. Its abilities are just too good to ignore and this should be a really fun card to brew around.