Well, lets see, from a manufacturers point of view if you allow used sales to happen on your site then chances are you are going to sell fewer new products which is what they are in business to do. Kind of makes sense. There are lots of other places to look for used products on the web.
If you have the room for a vertical center then by all means do it, and if 3 of the same will fit, ie. 3 M60s, that would be even better, otherwise you do what you can when it comes to center channels.
I have used a M22 in combination with my M80s for a center and it had a very wide dispersion compared to the narrower dispersion of the VP series, but that can be said against any horizontal design vs vertical design. I haven't noticed any drop in SQ when off axis with the VP series just a drop in loudness. Other than that there isn't much difference.
The farther back from a horizontal center you sit the better, as it will allow the dispersion of the center to make it out wide enough to cover all seating positions.
