Vandersteen VSM-1 Rear Speaker Location

Assuming I have full flexibility in speaker placement, where is the best place to mount the Vandersteen VSM-1s? I can put them either behind the main listening location (about 6 feet) or to the side of the main listening location.
DMinches- I met Richard Vandersteen for the first time this past Monday at American Audiophile on Long Island. I had several questions that I wanted to ask him including the very one you are asking. Richard indicated that he prefers, and has mounted, his VSMs behind the listening area. Without going into details, he has tried both side and back locations, with back wall placement giving the best results.
I put mine up on a audio stand that just happened to be the right height, just above ear level and tried them in several places. I started with them directly to the sides and then kept moving them back and kept going until they were all the way behind me on the back wall, about 8 feet from the listening area.
My experience was that they sounded best along the side wall where I could see them by comfortably turning my head. I don't know how many degrees that would be but, it works.

You will get great effects both ways. The problem is that it's room dependant as to which you'll prefer. If you can get stands or friends over to help you move them around, great. Otherwise, I'd put them where they fit on the walls best and not worry too much about it. Mine are mounted on the sides and they work out really well. I would prefer them further back, but can't do so in my room. If you can, then place them as headphones and then try moving them back.

Good luck!