Okay, I think I got it to work. But I mistakenly did two things without determining if that was the problem. But anyways,
A. I first installed K-Lite Codec Pack (their "Mega" installer, and "Advanced" profile). With the CriticalZoneV2 theme, background videos did not work using WMP or VLC.
B. For some reason I had remembered that under Windows 10, Apps & Features > Programs And Features > "Turn Windows features on or off" I had disabled Media Features which houses the native WMP. Not sure why I did this, but I re-enabled it. Back into BigBox, the videos now play using VLC
Not sure if the latter was directly related to this issue, or if K-Lite solved something, but it seems to be working fine now. Sigh, want to apologize for taking up your guy's time with what probably is my blunder. Thanks so very much again for the help, all!