Axioms and Rockets are in the same ballpark when it comes to sound quality. Many of the newer Rocket models are not that different from the Axioms (I have personal experience with Axiom M60s/VP150/QS8s AND Rocket 850s/RSC200 Bigfoot center/RSS300s). However the 750s, which are being discontinued and are on sale at a knockout price, are probably the one Rocket speaker that sounds least like the Axioms. IMHO, any of the Axioms would be a better match for the VP150.
As Ken mentioned, Rocket does have a smaller center, the RSC100 (10.1" H x 8.4" D x 20.5" W • 25 lbs each), which is on sale for $199, but are backordered and due at the end of the month.
Just my $.02, but I'd either go all Axioms, or all Rockets, rather than mixing them. I doubt you'd be disappointed in either system.
