I'm not sure if this is a bug or not, as there's limited info on what types of images we're actually targeting to fix or not with vips scalar.
can't be scaled using Special:VipsTest. If you have bilinear off, it returns error: 'Error creating thumbnail: vips warning: shrink: not integer shrink factors, expect poor results'. With bilinear on, it just does "Error creating thumbnail:".
Given that non-progressive jpegs work fine in our current setup, if vips doesn't work with progressive jpegs, I'm not clear what the benefit is over the current image magick set up for jpeg files.