Sure sst, some use a vertical center speaker such as the M2, M3 or M22 because of the generally wider and smoother horizontal dispersion(as shown by studies which have been cited here)than the horizontal MTM arrangement which is frequently employed as a center speaker. It's been said that the TMMMT arrangement of the VP150 is significantly better than the MTM. In any case, a small vertical speaker having a response as similar as possible to the mains above 80Hz or so is a good choice. If you do have room for the M22 it would be preferable since it would match the M80s above the low bass a bit more closely. Either by way of its height or by tilting it back slightly, the tweeter of the center speaker should point at the listener's ears.
