High current z pinches are used for a variety of high energy density physics applications, including inertial confinement fusion research. Present state of the art in single-pulse drivers can deliver twenty megamperes through an imploding plasma load; upon stagnation on the axis the plasma radiates more than a megajoule of energy in a few nanoseconds. Emphasis on fast plasma implosions (one hundred nanoseconds or less) means that the driver must deliver megajoules into vacuum at several mega volts from a driver with impedance that is a fraction of an ohm. The upgrade to the Z driver requires high reliability operation with as much as 6 mega volts peak on the water-vacuum interface. Cost and inductance (efficiency) are important, and make highly stressed vacuum insulators desirable. This paper describes the requirements and design for the vacuum insulator of the upgraded Z driver. The insulator stack is required to deliver 4 megajoules of electrical energy to the load within one hundred nanoseconds. It is desired to operate the upgraded Z driver once every few hours; hence reliability of the vacuum insulator is a critical design parameter.