Abstract: Vacuum adapter apparatus, systems, and related methods for use with inflatable structures having relief valves are disclosed. An example vacuum adapter for a relief valve of an inflatable structure includes a first end portion adapted to connect with the relief valve, a second end portion adapted to connect with a vacuum fluid line, an intermediate portion coupled between the first and second end portions, a vacuum chamber extending through the intermediate portion between the first and second end portions such that a fluid is conveyable through the vacuum chamber from the relief valve to the vacuum fluid line, and a fluid seal positioned on the first end portion. The fluid seal is configured to engage an outlet portion of the relief valve to maintain a negative gauge pressure in the vacuum chamber that causes the relief valve to open during a deflating process of the inflatable structure.