As the One Show said, Vettriano copied from an artists' modelling book, containing depictions of the body in various postures in the correct proportions (cheaper than using live models, as he said ). Now, it may be that the couple were depicted mirror fashion so that the outline could be traced on tracing paper and transferred, the marks on the tracing paper being pressed face down ,thereby giving the image outline the right way around, but Vettriano didn't appreciate that and copied the image straight from the page. He then enlarged the outline using a pantograph as needed (or , alternatively,they were right originally but he used tracing paper !).
As a picture, it succeeds as what picture dealers and collectors call 'a conversation piece' , a picture which hints at some story.Working out what the story is or may be gives rise to conversation ( hence the name). As such, it works beautifully ..