It actually stands for "Vielseitigkeits-Sattel Dressur", which means General Purpose saddle, dressage. Its counterpart is the VSS, "Vielseitigkeits-Sattel Springen" (GP, jumping). I think the reasoning is that it could be useful for someone who mainly does one discipline, wants to dabble in the other, and doesn't want to own 2 saddles. But as tabithakat64 mentions, the VSD can be useful if the horse has big shoulders. I'm considering going down that route with the Spooky Pony at some point when I've not just shelled out for a dressage saddle for the mare.