It's basically photography, but with the idea of projecting the images you
film into the plane of a transparent surface, so that the user (typically
an animator) can draw a clear image of it.
I'd guess that the benefit for VMS study would be that, by placing a
rotoscoped image into the apparatus, the user could draw "over" the symbols
and images to make a cleaned-up copy.
Several hand-copies of the VMS were made in 1931: this is almost certainly
how they were done.