Check out this parody of a German propaganda film from 1941, which takes footage from Nazi rallies and edits it to a popular musical number from the period to make the German WWII troops look ridiculous:
It may not be of the vidding tradition per se, as that grew out of fandom in general and the fanfiction tropes and traditions specifically, but I think it certainly does constitute one of the earliest video remixes I’ve ever seen. Fascinating!!
Thanks to my colleague Ruth Walker for pointing this out to me.