Hi Dave!
could you please check the color of the LEDs of the board with the VFD? Because I don't really think that the yellowish soldermask has been caused by the UV coming from the LEDs (you should see at least some sort of shadows around the LEDs). Even blue LEDs emit a very very very negligible amount of UV (it's almost on the noise floor of our spectrometers). Longer wavelenghts LEDs (green, yellow, red) emit really 0 UV (well... maybe some photons per second
).
Only some very recent white or other phosphor converted LEDs could emit some traces of UV, as some of them are using an UV LED (instead of blue one) with an optimized phosphor. But that piece of hardware seems to me quite old to contain such a new technology.
I think that, as you originally said in your video, UV came from the outside solar light (which, even though there is a quite thick plastic, some UV could still penetrate and reach the soldermask! Much more than any UV emitted from LEDs). And maybe that gaming machine was outside some pub or bar, and some reflected light could still reach that gaming machine.
Cheers!