Does anyone know what the purpose of the wing like appendages on the VSD?
Opinions vary. Some think they are purely for atmospheric flight and some think they are extra torpedo tube decks. I imagine some think they are both.
I believe that the original WEG version listed them as the torpedo decks, but obviously they are there so it looks different from the ISD at a glance.
The VSD has a fun history. In the Star Wars universe the VSD predates the ISD, created by the father of the inventor of the ISD, who the turned rebel. In reality it is the other way around. Lucasfilm created the ISD, WEG needed to expand the universe with other older ships, so to differentiate the VSDs in the Star Wars RPG, West End Games gave them a slightly different look. Early expanded universe authors used WEG material as a writing aid to maintain basic continuity. The VSD has remained largely unchanged in appearance from those early WEG drawings.