Thanks for everyone's input so far! It's beginning to sound as though my "fears" regarding the VP100 properly matching the M50's are unfounded.
Regarding the M3 as center, I am using a CRT, and an M3 would need to be closer than the 6" distance so that probably rules out that route.
VP100 sounds like the way for me to go. My room is 14'D x 15'W, but the one end of the 15' wall is angled (think bay window, but it's the whole wall w/ 3 separate windows), and the other "wall" is my staircase upstairs (and it's fairly open, with just vertical columns from floor to stairs). So, it's not a square room.
Any comments regarding the VP100 being sealed vs. a rear-ported design? Am I right or wrong in thinking that the sealed VP100 should not require the breathing space behind the center like a rear-ported design?