A floor vault was a place to keep valuables such as gold-pressed latinum. It was difficult to break into and commonly used by entrepreneurs who wanted an inconspicuous place to hide their goods.
Quark had a floor vault to keep his latinum, which his brother Rom broke into on at least one occasion in order to store the extra profits Quark's Bar earned while Quark was on a business trip. Rom replaced the damaged parts out of his own salary, and Quark did not notice; however, when Quark learned of this "treachery," he became angry and claimed Rom could not be trusted. (DS9: "Necessary Evil")