I read an answer to what is inside a typical vacuum, and it suggested neutrinos from Space, and vapor from the container. So, maybe they add to this effect...
But why are photons released from high-voltage electricity passing through a near vacuum?
The 'vacuum' in this case is really a low pressure gas (with a pressure of 10 to 30 mm of mercury). The light show is caused by ionization of the gas by the electric current.