After 3 evenings of listening to something with my current setup...
2 pairs of M22 mains
here's my observation so far...
The M2 and VP160 is a poor tonal match. With the VP100 and VP150, I couldn't even tell that the M2 was working. With the VP160, it's obvious that the tonal quality of the M2 is much, much higher than the VP160. it gives certain voices a colored upper end sound (a tinny sound, if you will). An M3 might make a better match for my upper center channel, but I wouldn't know without trying one first.
The M22 and VP160 combination is working good together. That might be good news for those that want a VP160, but don't want to swap out their M22 mains. The VP160 adds a lot of missing dimension, especially for special effects where a loud noise gets blasted in the center channel. For music, it also adds in some mid bass that I was having a hard time hearing before.
So, overall, I'm quite pleased I've added the VP160 center channel to the mix.