I tend to agree that it doesn't look quite right but I've never seen it produced by a 747. My window overlooks the threshold of SDU and I frequently see 737s and A319s blasting up through thin low cloud. My view is from directly behind, exactly as in the photo here, as they bank left to skirt Sugar Loaf. You can really only see the vortices well when there's a contrasting background, usually darker. When you do, my recollection is they're broader and more prounounced as twins rather than what almost looks like a single cone in this photo.