受試者包括十名超級有錢人(Hollywood actors, business tycoons, scientists, artists, etc.)，他們每個人都有一台機器僕從以滿足他們的任何需求，而且在跟第二群人─擠在難受、擁擠的側翼的三百個一般人─交涉任何事都有優先權。所有事情都由富人裁判、所有處置都由機器僕從執行。原本這是要觀察經典的勞資衝突，但因為避難所側翼的建築工程完全停滯而出現變化。在避難所主要區域完工後，資金就被切斷了。主要金主伊斯拉·帕克編了一個沒辦法從客戶拿到錢的故事，終止了投資。The second wing was never finished and while the vault was used as a fallout shelter, the vault experiment never started.其次，測試群A（富人）的其中一人柏特·李格斯，前通用原子中負責進階機器人計畫的研究人員，他和他老婆說服其他A群的人，為了應對越來越緊繃的國際局勢，撐過戰爭最好的辦法就是把自己的大腦插進機器腦裡。All inhabitants and senior Vault-Tec staff agreed to this, against the protests of the overseer. 最後當大戰爆發時、避難所被封閉，而包括測試群B（勞動階級）在內的外者都無法進入避難所，因為保安程序被機器腦控制了，而監管人成為避難所內唯一的人類─一個充滿機器腦的避難所內，唯一的受試者。Shortly after, the Overseer was driven mad by the robobrains and committed suicide.
Upon entering the lower level of the hotel, the vault entrance will be found with the door sealed. Activating the door controls will trigger a conversation with Maxwell, who asks the player if they are the detective he requested. Answering yes will open the door. Maxwell will be waiting for the player in the front entrance. If Pearl is present due to recruiting the player as the detective and escorting the player, she will interject and answer instead.
Going right leads to an extremely lavish hallway with a large fountain. Straight from the fountain leads to the ballroom, where the murder took place. Right from the fountain goes to either Pearl's shop on the right, or residences ahead. This residence hall contains a room full of display cases containing various items attributed to Ezra Parker's pre-war exploits, which leads to Ezra Parker's bedroom; Keith and Gilda's stage, with their adjoining bedrooms; and the classroom, which functions as Bert Riggs' lab and contains several tools and a chemistry station. A staircase at the back of the hall leads to the upper level.
Going left from the fountain, the player can either go left into the security office housing four protectron charging stations, a desk, and a staircase to the upper level, or right leading to another hall of residences. This hall houses Santiago Avida's art gallery, next to it is a decrepit bedroom that appears to belong to Santiago, though it appears he never uses it. Across the hall is Julianna and Bert's room (filled to the brim with assorted junk) and at the end of the hall is the showers. Beyond the showers is the beach, with sand and an artificial lake.
Going left from the main entrance is a short hallway filled with trash that leads to a sort of parking garage. A Corvega is suspended from a crane surrounded by nine garage doors on either side, three doors to a level. The large vault elevator, similar to that of Vault 111 and seen on the surface outside the hotel, can be found here. A hole in the wall on the center garage, second level right side, leads to a small cave with a bulldozer and crates. Beyond that is another small room with a door blocked by crates.
The upper level of the vault is in complete disrepair as opposed to the lower level. The overseer's office is found upstairs in the right corner of the ballroom. Inside, a desk takes up most of the main room and contains the skeleton of the overseer, which is wearing a Vault 118 jumpsuit, and the overseer's terminal next to him. Behind the desk is an entrance to the overseer's personal quarters, with a lavatory. The only other location upstairs is the Hydroponics lab, across the ballroom from the overseer's office. A variety of plants can be found here, as well as at least one Concierge attending them.
Except where required for the Brain Dead quest, the NPCs in this vault cannot be killed. Causing any of the robots to become hostile will cause all of the robots to become hostile, but they will always replenish their health instead of dying. However, once the mission is completed, the NPCs can then be killed.
The vast majority of items in the vault count as owned and need to be stolen, should the Sole Survivor wish to acquire them.
Vault 118 is the first ever vault to make an appearance in an add-on expansion.
The water of the artificial beach is radiation free to stand in, but bottling it in survival mode still gives dirty water.
The player may use any bed to sleep.
If the player kills Pearl, none of its inventory is available to loot.
Despite usually being opposed to stealing, Piper may quip that the hotel's wealthy inhabitants arguably would not miss some of their "nifty tech" while exploring the vault. However, she will still lose approval if the Sole Survivor appropriates owned items.
The resident robobrains with the exception of Ezra Parker are the only non-homicidal robobrains in the game, and the only ones aside from Jezebel that can be talked to. While still comparatively sane, they are either in denial or unaware of the fact that the war they wanted to wait out lasted only a few hours and ended two centuries ago. There is no option to tell them.
The painting of the sailboat in the artificial beach may be a reference to the scene at the end of the film The Truman Show, where the titular character crashes a sailboat into the wall of an artificial ocean.
Upon leaving Vault 118 via the elevator, the player may get stuck within the elevator. Reloading a previous save might help, some report that attempting a fast travel plus a few minutes wait time in the elevator fix it. Hearing elevator music upon loading may be an indication that you are no longer trapped.[已核實]
An alternate fix is to press the 'Activate/Call' button for the elevator even if the elevator is at the basement level with the door open. Close doors, press the call button and then proceed to take the elevator as usual.[已核實]