Most vacuum vessels are bodies of revolution that are cylindrical or spherical. A square vacuum vessel was developed for a specialized application that needed to be suitable for repeated, rapid sealing and unsealing, and operation at mild vacuum (∼0.5 bara). This paper discusses the challenges that we faced in fabrication and testing of the vacuum vessel. The vacuum vessel was initially tested in a test rig. A second set of challenges was encountered when the vessel was tested mounted in the system. It captures the lessons learned from fabrication, rework, and modifications that were necessary to get the vessel to pass leakage testing.