I used an inverted vertical M2 as a center for a while. Worked really well -- the only reason I replaced it was that it looked a bit wierd on top of my 27" TV.
Lying the M2 on its side definitely was not as good. The issue is that you want woofer and tweeter to be in line horizontally so you don't get a blurred "image". If you have a symmetrical driver array (eg. woofer on each side of the tweeter) the woofer sound effectively comes from the center of the array, same point as the tweeter.
At least that's how I understand it
I don't know if I would say the M2 was better than the VP100 -- I think the extra woofer/mid on the VP100 made a difference although I think the M2 matched the M60s sonically better than the VP100 did.
If you can live with a vertical M22 I think that would give you the best of both worlds.
