VaultWiki picks the best image manipulation software available through your PHP installation for any given task, since some libraries are better at different kinds of images. It does not care about your XenForo setting. Basically, if you have Imagick installed, it will use it for some images.
I tried the closest Imagick and PHP versions to what you are using and was unable to reproduce the $resized error using the same image quoted in your error message. I was unable to upgrade ImageMagick past 6.9.12-93 without upgrading my server OS. So I wonder if there is a bug in newer versions, or if you possibly upgraded ImageMagick without rebuilding the PECL extension and it causes errors.
To force VaultWiki to use GD, you will have to edit
src/addons/vw/vw/_core/model/image/format/vw.php. Find:
Code:
public static function supports()
{
AFTER it, add:
If this works, then we know something is wrong with how ImageMagick is working on your server, and we'll have to add an easier admin option for VaultWiki.
The only other thing I can think of is bad sectors on your server's hard disk resulting in corrupt image files. But the image is mostly black which is something that ImageMagick does if certain operations are performed (like sharpening an image to the maximum possible amount). If the hard disk was damaged, I might expect random colors.