Why not set the VP180 as large as well? Why re-route the center bass to the mains when the center is just as big?
Since you in the design phase, you may want to look into making sure that the fireplace is as low profile as you can manage. I've seen a lot of fireplace TV setups as I look for a new house, and I've been dismayed that the architect builds huge fireplaces and then the TV is basically mounted on the ceiling. It's a terrible setup and I end up having to move on to another house that can accommodate my setup. Since you are building, make sure you find a fireplace that isn't too tall. I've seen some really slim fireplace designs that look fantastic, and still keep the TV off the ceiling.
