I have M50's and and older VP150 with a WTWTW array rather than the current TWWWT layout. I find the timber match to be very very good. Even though my VP150 is supposed to be less able in terms of even dispersion than the current version, I have never noticed any problem or differnce across the listening positions in my setup. From everything I have heard I beleive the VP100 is excellent as well. The reasons for selecting the VP150 would probaly be the size of your room, the distance from speaker to the listener, and the volume at which you like to listen.
Laying bookshelf speakers on their side will result in an uneven dispersal of frequencies on the horizontal axis due the the asymetric driver layout that would result. This may or may not be a problem depending on your room and listening positions. I believe in earlier threads Alan reported an M22 on its side worked quite well for him as a center.
Given you have M50's I think M3's would be the obvious choice for alternate center(s). I beilieve JohnK uses an M3 for a center channel and is very happy with it. I may be wrong about this, that he is using an M2 with M22 mains.
Using two bookshelves for a center in parallel will result in halving the resistance of their load which may or may not be a problem for your receiver depending on what it is and how loud you play it.
Personally, I would not worry much about timber matching and dispersal with a VP100 or 150. These problems seem to be by far the exception in most setups. Try it and if it really doesn't work you are only out the shipping for one speaker.
