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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?

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What actual example of photons being generated in a vacuum by high-voltage electricity are you thinking of? – Benjamin Franz Oct 23 '12 at 13:26
@BenjaminFranz Like in this video – Tom Oct 23 '12 at 13:29
Summary of above video: transparent tube (~.5 meters long, ~10 cm diameter) with embedded anode and cathode (each at opposite end, about 5cm from tube-end); hole at bottom of tube presumably to cause 'vacuum' at '29 inches of vacuum'; 10 kV DC passed between electrodes (see at 6:10 mins for effect). – Tom Oct 23 '12 at 14:13

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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.

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Good answer, thanks. – Tom Oct 24 '12 at 8:25

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