Rick,
The reason for the price difference between the M80 and VP150 is simple: the M80 is designed to be a full-range speaker, capable of down to ~30Hz, while the VP150 is designed as a "small" speaker in the THX lingo, capable only of ~80Hz and up. The bottom-octave reproduction always costs a lot, because it requires a larger driver (often more than one, as in the case of the M80) housed in a larger cabinet.
By the way, absolutely nothing is wrong if you use a third M80 as a vertical center channel (if you can place it nicely) -- in fact, three identical speakers would make the ideal configuration of the front L/C/R channels.
