How much energy is needed to form plasma, and is a vacuum necessary for plasma formation?
In general, the plasma may be a DC or AC one.
The plasma energy needed is required for at least 2 main items:
1- Just to start the plasma: which means the energy required to start the breakdown the resistance of the media between the plasma electrodes.
2- The energy needed to sustain the plasma after ignition.
For both of these tow main factors, there are some important factors such as the type of the gas between the electrodes, its pressure, the distance between these electrodes, the geometry and some others.
Of course one can produce a plasma in normal pressure, at reduced pressure, at high pressure and at vacuum.
I think the question is very broad to answer specifically.
In addition to Emad Alashkar 's serious answer, a bit into the fun fact range:
vacuum is not needed for having a plasma, you just need brutal conditions such as the gravity field of a star or Randall Munroe's relativistic baseball:
https://what-if.xkcd.com/1/