Maybe you are right. Perhaps he did. Could it have been an older photo?
You can always ask yourself if what Bridgland, Whife or Barney wrote between the 1930-50s is still relevant today. Even Roetzel says you should wear a white waistcoat. Who at a ball today would call out to a man to fetch a drink just because he is wearing a black waistcoat?
I've found the evening suit. It is here: http://www.metmuseum...D=80003360&vT=1
According to the page, it was made somewhere between 1938 and 1965 (although the 1965 part could just refer to the replacement trousers). In this picture (http://3.bp.blogspot...n white tie.jpg
) the DoW, looking quite old (so I'm guessing the photo dates from the mid-to-late 1960s) is wearing a white waistcoat. So it appears that the DoW made the switch some time between 1938 and the 60s. Perhaps he was a bit slow to change because, as a highly recognizable public figure, he was unlikely to be mistaken for a waiter?
Of course, you are probably right that the choice of waistcoat is a bit of a moot point. Even if one were lucky enough to be invited to a white tie event (they are quite rare these days), it would be unlikely that anyone else would be as thoroughly acquainted with the subtleties of formal evening dress to make the distinction.