Does the VP160 blend well with you M80's? If so, then keep the VP160, since it is a superior design for those sitting off axis. If you think that there may be issues where there isn't a seamless front soundstage, then the VP180 will probably improve on that a bit, since it uses identical drivers, cabinet size (and crossover?). Those are probably the two major issues to consider.
Also if you are not noticing any off-axis differences on how the VP160 sounds, then that is a good thing. It is definitely noticeable on a MTM design. I am planning to upgrade to the VP160 at some point, to be able to have a full range center with better off axis response.
