You are supposed to uncheck the box because you don't want to automatically crop images.
Thank you. I just interpreted this wrong. However.... when creating a slide show that option doesn't exist. At least I have found it so. The slide show doesn't resize them to fit the screen despite having that box unchecked and they're all too large. Maybe that's something Vegas can fix.
I did find an easy solution. I just restore one of the images in the slide show, copy it then select all the images and do a "paste event attribute".