Vault

Vaults are special rooms that contain a variety of pickups/chests (or occasional lack thereof). Vaults require two keys to be opened, unlike most other special rooms. For a Vault to spawn, the following has to happen.

An Arcade has to be able to spawn that floor. This requires that both:
- Isaac is holding five or more pennies
- The floor Isaac is going to is the second floor of a chapter
Once the game determines an Arcade will spawn, a Vault has a 1/10 chance to replace that Arcade. This chance increases to 4/10 if Isaac has two or more keys.
The floor Isaac is going to is the second floor of a chapter or the first floor of an alternate chapter ( Downpour, Dross, Mines, Ashpit, Mausoleum, Gehenna and Corpse)
If Isaac has 2 or more keys, a Vault has a 1/3 chance to spawn. This chance increases to 4/10 if Isaac has five or more pennies, and will replace the Arcade.

ID	Contents	Image preview
0	Two chests and a Red Chest.
1	Three Chests.
2	Four random pickups behind Key blocks and two Chests.
3	Three Locked Chests.
4	Two random pickups behind rocks, two Bombs behind key blocks, and three Bomb Chests.
5	Seven Red Chests behind spikes. The spikes surrounding the three chests in the center occasionally retract.
6	Ten random pickups, four of which are behind rocks.
7	An item.
8	Eight coins and a Golden Key.
9	A Key Master.
10	A Shopkeeper and two Spiders.
11	Four Shopkeepers and two Bombs.
12	Three trinkets.
13	Thirteen Poops.
14	A Chest, Locked Chest, Red Chest, and Bomb Chest.
15	A full Red Heart, Soul Heart, Nickel, Penny, Key, Bomb, pill, and card.
16	A Chest, two Lil' Batteries, and two random pickups behind rocks.
16	A Chest, two random batteries, and two random pickups behind rocks.
17	A Locked Chest and four Bombs behind spikes.
18	Two Chests and a trinket.
19	25 pennies and a Donation Machine.
20	Five Chests.
21	Eight Attack Flies surrounding a Corny Poop.
22	Three Chests.
23	Three Chests and some cobwebs.
24	Seven Pennies and a Shell Game.
25	A Shopkeeper, a Key, and two Locked Chests.
26	A Dime.
27	Two Keys, two Chests, and a Locked Chest behind a key block and iron blocks.
28	A Shopkeeper, five Pennies, two Double Pennies, and three random coins.
29	Three runes.
30	Three Reward Plates.
31	Nine random pickups.
32	Five Lil' Batteries.
32	Five random batteries.
33	Greed, two Golden Poops, and four Shopkeepers.
34	A Key Ring, Double Heart, and Double Penny.
35	Two Chests and a Reward Plate.
36	Three Chests behind a key block and iron blocks.
37	Three Spiked Chests.
38	An Eternal Chest and two Keys.
39	Five Grab Bags and four random coins.
39	Five random bags and four random coins.
40	Fifteen Locked Chests.
41	A Grab Bag, three Attack Flies, and a Poop.
41	A random bag, three Attack Flies, and a Poop.
42	A Devil Beggar, a Bomb Bum, and a Beggar.
43	Two Chests, and a Haunted Chest.
44	Four random coins, and three Black Sacks.
45	Two Grab Bags, and three Black Sacks.
46	Four Black Sacks around spikes.
47	A Mega Chest.
48	Eight random coins, a Beggar, a Battery Bum, and a Rotten Beggar.

Notes

The texture Vaults use for their rooms are the same as Chest.
Dad's Key, Get out of Jail Free Card, and Mr. ME! open Vaults in one use, despite the fact that they require two keys.
Many vaults contain locked chests or key blocks, further increasing the required number of keys to fully utilize them. Make sure you have enough keys if you choose to enter one.
Due to how Vault generation works, a floor cannot have both an Arcade and Vault on it; they are mutually exclusive.
- If an Arcade is especially valuable to your run, it may be worth limiting your keys to one going into a floor with an Arcade on it to reduce the chances of it becoming a vault.