I think you'll be ok with setting the X-over at 100 or 120 Hz for the VP. Try running some test tone sweeps and note if you can hear a dip in the volume at that 100-200 Hz range (or use an SPL of course).

Alternatively, do you have room for an M2 or M22 as a center? That could save you some cash and possibly improve your SQ as well. Just a thought instead of contemplating the VP150 upgrade.

Dan
On-Wall M5HP LCR, QS8 & EP500 in 7.1