Originally posted by Jasvox My Tamron version does the same thing. It is the weight of the lens elements and gravity, who can fight that? Think of it, its an 18 to 250mm lens in a barrel about the same size as the kit lens, I can live with the occasional zoom creep when its pointing down. Once you get used to "locking" it when not shooting, it's not an issue at all.
As for the vignetting, just learn to find the lens' sweet spot with the aperture setting and vignetting usually isn't such a bother or issue.
Jason
You can ask the camera to use the lens' sweet spot by setting your program line to MTF. Shoot for a day in this program line, and then check the aperture settings the camera used. My 50-135 uses f/4-f/5.6 when it can, for example.