The center channel is the most critical speaker for HT use, so coupling M3's with a VP150 is fine. I don't think you'll ever regret going with a "better" center channel speaker.
You could also go with a 3rd M3 (or even another pair of M3's) as center. If you've never head a HT system with identical speaker all around, you'd be in for a treat. I've done this with my Titans, and a friend and I thought it sounded better in some ways than his Studio 100's with Titans in the rear, just due to the better blending of all the channels.
M3's vs VP150 - really your choice. If you have a direct view display, then the VP150 may be a better choice due to the better magnetic shielding in the VP line. I'm planning on going VP150 with my M50's since "Santa" just gave me a voucher for an Axiom center channel!
Regarding the M50's - I love mine. BUT, for HT use, they do not put out the lowest frequency "impact" bass you may want. they go low, but they don't pressurize the room like a true sub will. For music, I feel they offer very good bass. Also, though the M50's sound fine at lower volume levels, they really come alive once that knob is rotated clockwise a bit...
