White Christmas.

Can't say for sure about the copy you watched, but you might not have missed any picture area in full screen. VistaVision was a flexible system that was designed to be shot in aspect ratios ranging from the standard 4:3 to as wide as 2:1, and could be exposed either with an aperture plate to capture only a selected aspect ratio, or full frame with a "safe area" within it for projection screen masking.

VistaVision was more about image quality than picture size or shape. By running the negative horizontally through the camera rather than vertically, with a frame eight-perforations wide instead of one four perfs high, it exposed twice the picture area of standard 35mm, and yielded a finer-grained image when printed down to standard 35 for theaters. 

     

Paramount's sales pitch to exhibitors was that it required no special equipment or modifications, but it never caught on with other studios, and only a handful of non-Paramount VistaVision titles were produced before the process was retired as a production medium in 1961. But because of the superior quality of the images it produced, it remained in use by special effects houses for decades to accomplish complex composite shots with minimal loss of image quality. 

When first mastered for home video, some VistaVision titles were done full frame even though they'd been designed to be screened with only the safe area visible. An early home video release of North By Northwest (one of only two VistaVision features made at MGM) was an example, and some shots revealed the tops of sound stage sets that were never meant to be seen by audiences.  

EDIT: Just for fun, here's a clip of scenes from that film with the proper aspect ratio highlighted at the center, but revealing the areas seen beyond the safe area in full frame.