As Ian has said, the VP150 I had matched well with the M22's as all the drivers match-up. I upgraded to the VP180 (but kept the M22's) to get a little more (ok a lot more) oomph for movies.
No need to trade in the M22's for M5's unless you have a the budget for it and were unhappy with the sound (though that would likely still cost you an extra $3500 for the upgrade + another $970 (all prices Canukistan $) for the VP-160).
But brwsaw is right, you could get another M22 as a center if you have the room for it under/over the display kept in it's vertical form. It is what Dolby recommends ;-)
