[akiselev]

An insulator by definition is matter that doesn't have freely flowing electrons so vacuums aren't an insulator. However, all insulators have a "breakdown voltage" at which point the electrons start "ripping" through the material so a vacuum has all of the properties of an insulator except the mass: the breakdown voltage is the the amount of energy required to eject electrons from the materials surrounding the vacuum and until you reach that point, no current will flow through the vacuum.