The graphics model used by VMD assumes the eyes looking in front of the viewer and focusing at the same point the focal length away. If the focal length is 0, the viewer's eyes are crossed and looking at each other. A larger focal length will often help in creating a viewable image.
The two parameters can be changed with the text commands display focallength and display eyesep, or using the Display form.
In general, try to make the eye separation as large as possible without giving the viewer a migrane, and try to vary the focal length to cut down on double images. It may often help to translate the molecule forward or backward and also adjust the scaling, since there is typically an optimum position for a molecule for a given set of stereo parameters.