I started out with the bookshelf versions of the M22/Vp100 and thoguht think they match quite nicely.
Having said that and as Chris(ClubNeon) mentioned the best match is another speaker exactly like the main L/R. If that is not an option then I would move to the next smallest vertical speaker, in your case the M2 and if that still won't fit, I would recommend the VP150,(in my testing of the freestanding versions(v2's) the VP150 played male voices a tad better than the VP100 but it was only really noticable with A/B testing) and finally the VP100 represents a very good budget horizontally designed center channel.
The resoning behind the suggestions for a vertical center is that this vertical alignment of drivers offers better horizontal dispersion of the sound than horizontally designed speakers like the VP series for an improved off axis listening experience.
