Vignetting is not a concern with a short hood. For a hood to vignette on an 800mm lens, it would need to be around 400mm long, so not an issue on an 800mm lens.
Longer hoods keep water off the front element when shooting in rain, snow, or water spray. If you only shoot with the sun at your back a short hood is not likely any concern. Once direct light starts to land on the front element the image can start to wash out, so the shorter you go the greater the likelihood that this can happen. Coatings on new lenses are very good, so that helps mitigate the issue.