if vertical isn't a problem, you feel a M80 > M22 > a vp160 or vp180?
Glad to see the center is being considered carefully - I think I may have opened a can of worms with all the options being discussed - sorry about that!
However IMO, the vertical M80 would be the best center channel matched with M80's L&R. 3 identical speakers across the front is the ideal situation. However once you go beyond that I think it becomes shades of grey and only listening in your environment can tell you what would be best. To summarise:
M80 center - PRO - best sound option, CON - worst WAF
VP180 - PRO - simply a horizontal M80, CON - horizontal array
VP160 - PRO & CON - blurs the line a little between horizontal and vertical arrays
M22 - PRO - vertical array, CON - can't carry as much lower frequency, not sure of sheer output capability as compared to VP180 or 160
dual M22's - I am out of my depth there and have no opinion, although I am intrigued