I would just be repeating a lot of what was said already, but I went from a VP150 to a VP180 and absolutely love its performance with my M60 fronts.
That being said, the VP160 wasn't even announced when I pre-ordered the VP180, and while my current theater was still under construction, I didn't want to miss out on the deal price. Had I know that the VP160 was coming out, I would have jumped on that and saved a few bucks, plus had a (arguably) better center channel design in the VP160.
I would get the better center first before any other speakers. More subs in multiple locations in the room is going to yield better subwoofer performance throughout the room vs. one newer or a bigger sub and having peaks and nulls throughout.
Plus what everyone else said....
