I had to read this thread twice (and the one referenced in the first post once). Yhe m2 sounds better than the vp150. wow.
I get that people are saying the vertical arrangement is better - but what about the fact the vp150 has 3 more speakers? I guess they don't come into play?
I am seriously considering picking up some mains and a center in the next month or so. That is about a $260 savings over a vp150.
What about using an m3 instead of an m2?
The extra drivers in the VP150 help give the speaker a higher sensitivity rating than the M2. However the M2 can reach lower than the VP150 and has the advantage of better horizontal dispersion than the VP150.
As I said originally, the VP150 is not a bad speaker by any means, it's just that the M2 performed better in my room with my setup - YMMV.
The main thing with a center (vertical or horizontal) is that it tonally match your left and right channels. I'm simplifying this, but both the VP150 and M2 use the same drivers as the M22's which are my main L&R channel speakers. An M22 would be even more ideal as a center but that is just not a physical option for me. So to answer your question, I would only use the M3 as a center channel if your L&R channel were also M3's (or maybe the M50's which I believe use the same drivers as the M3) so that they are all closely tonally matched.
Given what I now know, I would definitely make extra effort to accommodate a vertical center channel speaker (or two as Jay suggested).