thats a serious fire hazard
Not really. It is a fire hazard only in places where construction materials are flammable and allow fire to spread. Vaults are all concrete and metal; they lack the elements that make fire spread: carpeting, wood paneling, fake ceilings, cloth furniture, cloth cubicles, etc... In the US, a fire would destroy a house because it is all made of wood, or engulf a building because it is all carpeted, has fake ceilings, wood paneling, etc. so a fire can spread. In places like Mexico, where houses are made of cement and steel, a fire may burn the curtains and the sofa, but wouldn't be able to spread to other rooms.