A vacuum is the closest thing to a perfect conductor.
When we place an electric voltage on them, no electricity begins flowing. There are no currents or conductivity in a vacuum.
It can be made conductive by injecting electrons into the vacuum. It’s no longer a vacuum. A cloud of electrons can be described as a conductor.
However, even the perfect vacuum can be “broken down” at very high voltages into matter particles and antimatter. Look up “pair production.” A perfect vacuum is a non-conductor that works at low voltages, but not at extremely high voltages.