I had the same issue mounting my VP180 on-wall. I ended up mounting a 1/2" x 8" x 20" piece of plywood to the wall then attached the speaker mount to the plywood.
Yeah, if I didn't have the extra TV mount, I would have strongly considered doing that, however, the mount enables me to get the speaker a little closer to the wall and frankly even though the supplied documentation indicated as such, when I talked to Deb at the Axiom HO, she indicated the literature is incorrect and the 5 way binding posts apparently have never been part of the on-wall center channel design.
In any case, she passed on my issue to the "powers that be" and they will discuss options for a hopefully more flexible mount.
