You would certainly have enough room between the rear wall and the back of the 3A Sig's (my pair are about 30" out from the rear wall), but 18" from the side walls probably isn't enough. However, most room placement guidelines recommend that speakers NOT be placed the same distance from the rear wall as the side walls.
It's hard, however, to generalize a lot about room acoustics, since each room can vary due to furnishings, floor coverings, ceiling height, etc.
It's true that Vandy 3A Sig's are somewhat sensitive to room placement, but not acutely so. My best suggestion is for you to borrow a pair from your dealer for a home audition. That will give you a chance to vary the placement of the speakers and determine what is best for your room.