In the general dumb questions department, would there be any reason why OP could not start by turning the VP150 vertically ? That should address most of the MTM concerns immediately, shouldn't it ?
For what it's worth, my first thought was "M5HP" for the center because it sounds very similar to the M60, would have a decent chance of keeping up with power handling/SPL if you play at medium-high volumes, and is physically short enough that it might fit. A smaller speaker (M2/M3) seems like it might end up being too small unless you only run moderate volume, in which case M2 would probably be a better timbre match than M3 as long as you crossed over at ~100 Hz.
OP mentioned that they "might" be able to make an M2 fit leaving me to suspect space is an issue.
However, I agree that the OP should have fun first and turn the VP150 vertically first (even if it blocks the screen) and see if that produces immediate improvements. If so, the M5HP could be a decent upgrade if OP doesn't have 28" of room but 19" fits. Otherwise, an M2 or a VP160 needs 12" of space.
